update: getter to filter out possible duplicates

[ryuash]

add: test case

Description

fixed #44 As I was unable to reproduce the issue I don't know if this work around will be effective enough. Might need to refactor later on if each post has 1000+ likes.

Checklist

• [x] Targeted PR against correct branch.
• [x] Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• [x] Wrote unit tests.
• [x] Run flutter format.
• [x] Updated the documentation.
• [x] Added an entry to the CHANGELOG.md file.
• [x] Re-reviewed Files changed in the Github PR explorer.

[codecov[bot]]

Codecov Report

Merging #48 into master will increase coverage by 0.07%. The diff coverage is 80.00%.

@@            Coverage Diff             @@
##           master      #48      +/-   ##
==========================================
+ Coverage   18.14%   18.21%   +0.07%     
==========================================
  Files         245      245              
  Lines        5964     5968       +4     
==========================================
+ Hits         1082     1087       +5     
+ Misses       4882     4881       -1     

Impacted Files	Coverage Δ
lib/entities/posts/post.dart	85.39% <80.00%> (+1.86%)	:arrow_up:

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