Fix issues with duplicated MatrixEvent objects around threads

[t3chguy]

Fixes element-hq/element-web#21466
Believably fixes element-hq/element-web#21533

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This PR currently has no changelog labels, so will not be included in changelogs.

Add one of: T-Deprecation, T-Enhancement, T-Defect, T-Task to indicate what type of change this is plus X-Breaking-Change if it's a breaking change.

[germain-gg]

Otherwise looks good to me!

[germain-gg]

LGTM 👍 thank you for the tests

[codecov-commenter]

Codecov Report

Merging #2256 (8fd3d24) into develop (6192325) will increase coverage by 0.16%.
The diff coverage is 77.14%.

❗ Current head 8fd3d24 differs from pull request most recent head 6b63ddd. Consider uploading reports for the commit 6b63ddd to get more accurate results

@@             Coverage Diff             @@
##           develop    #2256      +/-   ##
===========================================
+ Coverage    58.38%   58.55%   +0.16%     
===========================================
  Files           91       91              
  Lines        16313    16285      -28     
  Branches      3763     3758       -5     
===========================================
+ Hits          9525     9535      +10     
+ Misses        6788     6750      -38     

Impacted Files	Coverage Δ
src/models/thread.ts	53.91% <50.00%> (-2.13%)	⬇️
src/models/room.ts	54.65% <70.00%> (+2.52%)	⬆️
src/client.ts	36.65% <100.00%> (-0.03%)	⬇️
src/event-mapper.ts	95.45% <100.00%> (+1.33%)	⬆️
src/models/event.ts	66.58% <100.00%> (+0.24%)	⬆️
src/@types/event.ts	100.00% <0.00%> (ø)