prevent double counting from same endorser

[CoderZhi]

different consensus vote topics from the same user were counted twice

[codecov]

Codecov Report

Merging #297 into master will increase coverage by 0.21%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #297      +/-   ##
==========================================
+ Coverage   64.49%   64.71%   +0.21%     
==========================================
  Files         114      114              
  Lines       10720    10721       +1     
==========================================
+ Hits         6914     6938      +24     
+ Misses       2906     2884      -22     
+ Partials      900      899       -1

Impacted Files	Coverage Δ
endorsement/endorsementset.go	54.38% <100%> (+25.81%)	⬆️
network/pinger.go	50% <0%> (+37.5%)	⬆️

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