User 1 copies a page X from 'channel1' to 'channel2'. As a result, in channel2, it happens that a new hst:template 'foo' is added. User 1 does not yet publish his changes
User 2 now copies also page X from 'channel1' to 'channel2'. He should now get a 'already locked exception' for the hst:templates even though the template 'foo' already exists!
The reason User 2 needs to get an exception, is that if he published his changes before User 1 does, his live page will miss the hst:template which was required for the copy of page X