only create a single session for duplicate Initials

[marten-seemann]

Fixes #2579.

[codecov-commenter]

Codecov Report

Merging #2580 into master will increase coverage by 0.02%.
The diff coverage is 83.78%.

@@            Coverage Diff             @@
##           master    #2580      +/-   ##
==========================================
+ Coverage   86.22%   86.24%   +0.02%     
==========================================
  Files         122      122              
  Lines        9603     9615      +12     
==========================================
+ Hits         8280     8292      +12     
  Misses        985      985              
  Partials      338      338              

Impacted Files	Coverage Δ
client.go	79.65% <ø> (ø)
server.go	83.69% <77.78%> (+0.10%)	⬆️
packet_handler_map.go	78.61% <100.00%> (+1.12%)	⬆️

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