If you send "TOPIC #chan :foo" twice (with the same "foo") then existing server implementations disagree very much on whether it should be echoed/transmitted:
bahamut and insp don't echo or transmit
charybdis/hybrid/plexus4/solanum, ergo, and unreal transmit (and might echo too, i didn't test)
irc2, ircu2/nefarious, and ngircd don't echo but transmit
If you send "TOPIC #chan :foo" twice (with the same "foo") then existing server implementations disagree very much on whether it should be echoed/transmitted:
Tested with https://github.com/progval/irctest/pull/231