diff --git a/CHANGELOG.md b/CHANGELOG.md
index 022d602..a2c162d 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -25,7 +25,9 @@ Due to the removal of legacy code, there are a few breaking changes in this new
 
 ### New Features
 
-_None_
+* Throw an error if a format string has mismatching types for the same placeholde position.  
+  [David Jennes](https://github.com/djbe)
+  [#44](https://github.com/SwiftGen/SwiftGenKit/issues/44)
 
 ### Internal Changes
 
diff --git a/Sources/Parsers/StringsFileParser.swift b/Sources/Parsers/StringsFileParser.swift
index 98e81a8..c7ceaf7 100644
--- a/Sources/Parsers/StringsFileParser.swift
+++ b/Sources/Parsers/StringsFileParser.swift
@@ -10,6 +10,7 @@ import PathKit
 public enum StringsFileParserError: Error, CustomStringConvertible {
   case failureOnLoading(path: String)
   case invalidFormat
+  case invalidPlaceholder(previous: StringsFileParser.PlaceholderType, new: StringsFileParser.PlaceholderType)
 
   public var description: String {
     switch self {
@@ -17,6 +18,8 @@ public enum StringsFileParserError: Error, CustomStringConvertible {
       return "Failed to load a file at \"\(path)\""
     case .invalidFormat:
       return "Invalid strings file"
+    case .invalidPlaceholder(let previous, let new):
+      return "Invalid placeholder type \(new) (previous: \(previous))"
     }
   }
 }
@@ -44,7 +47,7 @@ public final class StringsFileParser {
     }
 
     for (key, translation) in dict {
-      addEntry(Entry(key: key, translation: translation))
+      addEntry(try Entry(key: key, translation: translation))
     }
   }
 
@@ -81,8 +84,8 @@ public final class StringsFileParser {
       }
     }
 
-    public static func placeholders(fromFormat str: String) -> [PlaceholderType] {
-      return StringsFileParser.placeholders(fromFormat:  str)
+    public static func placeholders(fromFormat str: String) throws -> [PlaceholderType] {
+      return try StringsFileParser.placeholders(fromFormat:  str)
     }
   }
 
@@ -104,8 +107,8 @@ public final class StringsFileParser {
       self.init(key: key, translation: translation, types: types)
     }
 
-    public init(key: String, translation: String) {
-      let types = PlaceholderType.placeholders(fromFormat: translation)
+    public init(key: String, translation: String) throws {
+      let types = try PlaceholderType.placeholders(fromFormat: translation)
       self.init(key: key, translation: translation, types: types)
     }
   }
@@ -133,7 +136,7 @@ public final class StringsFileParser {
   }()
 
   // "I give %d apples to %@" --> [.Int, .String]
-  private static func placeholders(fromFormat formatString: String) -> [PlaceholderType] {
+  private static func placeholders(fromFormat formatString: String) throws -> [PlaceholderType] {
     let range = NSRange(location: 0, length: (formatString as NSString).length)
 
     // Extract the list of chars (conversion specifiers) and their optional positional specifier
@@ -178,6 +181,10 @@ public final class StringsFileParser {
           while list.count <= insertionPos-1 {
             list.append(.unknown)
           }
+          let previous = list[insertionPos-1]
+          guard previous == .unknown || previous == p else {
+            throw StringsFileParserError.invalidPlaceholder(previous: previous, new: p)
+          }
           list[insertionPos-1] = p
         }
       }
diff --git a/Tests/SwiftGenKitTests/StringParserTests.swift b/Tests/SwiftGenKitTests/StringParserTests.swift
index 30ab713..5eb7a0d 100644
--- a/Tests/SwiftGenKitTests/StringParserTests.swift
+++ b/Tests/SwiftGenKitTests/StringParserTests.swift
@@ -8,56 +8,72 @@ import XCTest
 import SwiftGenKit
 
 class StringParserTests: XCTestCase {
-  func testParseStringPlaceholder() {
-    let placeholders = StringsFileParser.PlaceholderType.placeholders(fromFormat: "%@")
+  func testParseStringPlaceholder() throws {
+    let placeholders = try StringsFileParser.PlaceholderType.placeholders(fromFormat: "%@")
     XCTAssertEqual(placeholders, [.object])
   }
 
-  func testParseFloatPlaceholder() {
-    let placeholders = StringsFileParser.PlaceholderType.placeholders(fromFormat: "%f")
+  func testParseFloatPlaceholder() throws {
+    let placeholders = try StringsFileParser.PlaceholderType.placeholders(fromFormat: "%f")
     XCTAssertEqual(placeholders, [.float])
   }
 
-  func testParseDoublePlaceholders() {
-    let placeholders = StringsFileParser.PlaceholderType.placeholders(fromFormat: "%g-%e")
+  func testParseDoublePlaceholders() throws {
+    let placeholders = try StringsFileParser.PlaceholderType.placeholders(fromFormat: "%g-%e")
     XCTAssertEqual(placeholders, [.float, .float])
   }
 
-  func testParseFloatWithPrecisionPlaceholders() {
-    let placeholders = StringsFileParser.PlaceholderType.placeholders(fromFormat: "%1.2f : %.3f : %+3f : %-6.2f")
+  func testParseFloatWithPrecisionPlaceholders() throws {
+    let placeholders = try StringsFileParser.PlaceholderType.placeholders(fromFormat: "%1.2f : %.3f : %+3f : %-6.2f")
     XCTAssertEqual(placeholders, [.float, .float, .float, .float])
   }
 
-  func testParseIntPlaceholders() {
-    let placeholders = StringsFileParser.PlaceholderType.placeholders(fromFormat: "%d-%i-%o-%u-%x")
+  func testParseIntPlaceholders() throws {
+    let placeholders = try StringsFileParser.PlaceholderType.placeholders(fromFormat: "%d-%i-%o-%u-%x")
     XCTAssertEqual(placeholders, [.int, .int, .int, .int, .int])
   }
 
-  func testParseCCharAndStringPlaceholders() {
-    let placeholders = StringsFileParser.PlaceholderType.placeholders(fromFormat: "%c-%s")
+  func testParseCCharAndStringPlaceholders() throws {
+    let placeholders = try StringsFileParser.PlaceholderType.placeholders(fromFormat: "%c-%s")
     XCTAssertEqual(placeholders, [.char, .cString])
   }
 
-  func testParsePositionalPlaceholders() {
-    let placeholders = StringsFileParser.PlaceholderType.placeholders(fromFormat: "%2$d-%4$f-%3$@-%c")
+  func testParsePositionalPlaceholders() throws {
+    let placeholders = try StringsFileParser.PlaceholderType.placeholders(fromFormat: "%2$d-%4$f-%3$@-%c")
     XCTAssertEqual(placeholders, [.char, .int, .object, .float])
   }
 
-  func testParseComplexFormatPlaceholders() {
+  func testParseComplexFormatPlaceholders() throws {
     let format = "%2$1.3d - %4$-.7f - %3$@ - %% - %5$+3c - %%"
-    let placeholders = StringsFileParser.PlaceholderType.placeholders(fromFormat: format)
+    let placeholders = try StringsFileParser.PlaceholderType.placeholders(fromFormat: format)
     // positions 2, 4, 3, 5 set to Int, Float, Object, Char, and position 1 not matched, defaulting to Unknown
     XCTAssertEqual(placeholders, [.unknown, .int, .object, .float, .char])
   }
 
-  func testParseEvenEscapePercentSign() {
-    let placeholders = StringsFileParser.PlaceholderType.placeholders(fromFormat: "%%foo")
+  func testParseDuplicateFormatPlaceholders() throws {
+    let placeholders = try StringsFileParser.PlaceholderType.placeholders(fromFormat: "Text: %1$@; %1$@.")
+    XCTAssertEqual(placeholders, [.object])
+  }
+
+  func testParseErrorOnTypeMismatch() throws {
+    do {
+      _ = try StringsFileParser.PlaceholderType.placeholders(fromFormat: "Text: %1$@; %1$ld.")
+      XCTFail("Code did parse string successfully while it was expected to fail for bad syntax")
+    } catch StringsFileParserError.invalidPlaceholder {
+      // That's the expected exception we want to happen
+    } catch let error {
+      XCTFail("Unexpected error occured while parsing: \(error)")
+    }
+  }
+
+  func testParseEvenEscapePercentSign() throws {
+    let placeholders = try StringsFileParser.PlaceholderType.placeholders(fromFormat: "%%foo")
     // Must NOT map to [.float]
     XCTAssertEqual(placeholders, [])
   }
 
-  func testParseOddEscapePercentSign() {
-    let placeholders = StringsFileParser.PlaceholderType.placeholders(fromFormat: "%%%foo")
+  func testParseOddEscapePercentSign() throws {
+    let placeholders = try StringsFileParser.PlaceholderType.placeholders(fromFormat: "%%%foo")
     // Should map to [.float]
     XCTAssertEqual(placeholders, [.float])
   }