XP is already reduced as you kill more of a type of monster, though, and they get more experienced as you kill more.

This is closely related to the so-called Uberjackal problem. In the case of that problem, the experience of the monsters increase, as Alucard noted, and the result is that monsters that are summoned a lot, like rats, jackals, etc, start to become well overpowered if you find yourself in the wrong situation. The XP/Loot situation is essentially a counterpoint to that one, an exploit rather than a complication.

I think the obvious solution is to have clones/summons treated differently from the monsters that summoned/bred them. In other words, for claw bugs for example, the original claw bug would be treated as a regular claw bug... any new claw bugs that originate from that original claw bug would be treated as a bred claw bug for all purposes.

Bred/summoned/etc beings should give out less experience, they should be far less likely to drop loot, and they shouldn't count towards the increase in experience of monsters of the same type (or perhaps they should count at a much reduced rate - say, 1/10th the rate of a normal being). So if you've killed 30 regular claw bugs and 80 duplicated ones, then it would be like you've killed 38 regular claw bugs as far as claw bug experience is concerned, rather than the 110 that it currently uses.

By not dropping experience/loot to zero entirely, you ensure that players don't find themselves fighting off hordes, fighting for their life, with no reward except survival. But they also can't just exploit the summons/clones for experience or loot. If they gave one tenth of the kill-count experience, but one 20th of the actual experience and one 20th of the loot, then you'll be better off killing regular beings than trying to exploit the cloning/summoning processes.

This should also apply to corpse drops, which should help to curb the blink dog teleport control balance issue.

I believe bred monsters (claw bugs, worms) already drop nothing. Gremlins are a special case, because they need help from water to duplicate.

Why should a cloned monster give a lower XP if it's as difficult to kill?

The loot thing just needs to be spread to multiplied gremlins, since it already affects breeders. XP is already dealt with by the current game mechanics.

Some of the suggestions have been added due to other issues fixed. Thus I'm closing this one and we'll see how the next version of ADOM feels.