BUG: old session deleted after saving to a new session (with a new name)

[milanglacier]

Bug describe

aichat -s

##### some conversation

.save session hello

aichat -s hello

 #### some conversion

.save session hello-2

Found that hello.yaml was deleted and hello-2.yaml was created.

Correct behavior

A new session hello-2.yaml is created but hello.yaml should leave unchanged (this is the behavior before 1.21).

[sigoden]

This is expected behavior.