fix #H4144 - rejecting named monster's own name

Some monsters can't be named, but if the user tried to assign them a
name that matched what they were already called, the rejection message
could be silly.  Reported case was "I'm Izchak, not Izchak!".  The fix
is more general than just for shopkeepers, although their reject
message was silliest when complaining about the name already in use.
For the cited case, feedback will now be 'He is already called Izchak.'

doc/fixes36.1

@@ -65,6 +65,8 @@ Warn_of_mon wouldn't have been able to sense giant ants if any creature were
 	to have that ability, caused by using 0 instead of NON_PM for 'none'
 tone down energy vortex's drain energy attack
 fix message typo if tame mind flayer dies trying to eat Medusa's brains
+use alternate rejection message if attempting to name an unnameable monster
+	with the name it already has
 
 
 Platform- and/or Interface-Specific Fixes