cluster: Improved discovery duplication error messaging.
cluster: Improved discovery duplication error messaging.

Signed-off-by: Andrea Luzzardi <442ffe3ffbfd024afcf0b7f8b108d05bca3aae43@gmail.com>

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arm,gdevillele/docker.github.io,rchicoli/swarm,BSWANG/denverdino.github.io,jzwlqx/denverdino.github.io,bdwill/docker.github.io,moxiegirl/swarm,JimGalasyn/docker.github.io,LukasBacigal/swarm,shin-/docker.github.io,BSWANG/denverdino.github.io,denverdino/docker.github.io,sanscontext/docker.github.io,beni55/swarm-1,unodba/swarm,vlajos/swarm,srikalyan/swarm,krystism/swarm,joaofnfernandes/docker.github.io,riuvshin/swarm,anweiss/docker.github.io,gradywang/swarm,cpuguy83/swarm,denverdino/swarm,alexisbellido/docker.github.io,jeffjen/swarm,londoncalling/docker.github.io,docker-zh/docker.github.io,bdwill/docker.github.io,docker/docker.github.io,alexisbellido/docker.github.io,denverdino/docker.github.io,danix800/docker.github.io,phiroict/docker,sanscontext/docker.github.io,adouang/swarm,akerbis/armhf-swarm,alexisbellido/docker.github.io,docker/docker.github.io,LuisBosquez/docker.github.io,jdamick/swarm,doronp/swarm,joeuo/docker.github.io,rav121/swarm,vlajos/swarm,LuisBosquez/docker.github.io,rillig/docker.github.io,snrism/swarm,francisbouvier/swarm,ejholmes/swarm,sanscontext/docker.github.io,iaintshine/swarm,ricca742/swarm,jzwlqx/denverdino.github.io,jimmycmh/swarm,cgvarela/swarm,ezrasilvera/SwarmACLs,ezrasilvera/swarm,docker-zh/docker.github.io,rav121/swarm,schmunk42/swarm,abronan/swarm,denverdino/denverdino.github.io,troy0820/docker.github.io