Steps for reproduce:
1) have a template T1 with any discovery rule
for example LLD rule key - test.pl
2) create T2 template with full clone T1 -> T2
3) create host with name test and link T1 to it, save
4) try to link T2 to test host and have the error

Template "Template OS Linux" with item key "test.pl" already linked to host.

This message is not clear pointing where this key is already used on this host.
PFA screenshots