prevent main fragment being created twice

[Rich-Harris]

This feels like a bit of a cheeky hack, but it a) works and b) is fairly unobtrusive. Fixes #1063.

[codecov-io]

Codecov Report

Merging #1064 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1064      +/-   ##
==========================================
+ Coverage   91.67%   91.68%   +<.01%     
==========================================
  Files         123      123              
  Lines        4459     4460       +1     
  Branches     1438     1439       +1     
==========================================
+ Hits         4088     4089       +1     
  Misses        157      157              
  Partials      214      214

Impacted Files	Coverage Δ
src/generators/dom/index.ts	95.5% <100%> (+0.02%)	⬆️

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