Hi,
I see at least 2 reasons:
1/ if your client sends a DHCP request, he'll receives 1 answer per DHCP server. If he's smart, he'll drop the second and above. If he's not ... If the second or above DHCP server are stubborn, he'll resend the answer another time, ... not good
2/ Each DHCP will reserve his address to the client... not good. if your DHCP servers are somehow linked to DDNS servers, your host might even get a host entries with a non-existent ip addresses
I had such a conflict 2 weeks ago. The tricky part is that the incident looks like a physical shortcoming (faulty cable, faulty network card, etc...) and you get caught in the wrong debugging.