Preferred is static ip.
i'd advise only 1 DHCP server on a network, i hope that's not causing the duplicate IPs.
with right configuration it is possible to run more ofcurse..
DHCP leases don't work that way, they re-check validation every half a time.
for ex:
if your lease time is 8 hours, the computer is the owner of the IP for the next 8 hours.
after 4 hours (half the time) the DHCP validate the computer. In case still online - renews the lease time to 8 hours.
means, if u obtain a new IP at 8 AM, and turns the computer off at 4:05 PM.
obtain: 8 AM
renew: 12pm
renew: 4 pm
lease time then will be 8 hours from 4 PM >> 12 AM.
/EDIT if the computer is offline the timer is keep counting and again half the time that is left - recheck.
lease 8 hours
recheck 4 hours - offline
recheck 2 hours - offline
recheck 1 hour - online > back to 8 hours.
recheck after 4 hours.
EDIT/
if it's 24 hours you can already see.
in other words, lease time should be 8 days.
depends on the network setup and usage, those are just defaults \ recommanded.
(DHCP>Scope>Right click - Options > under General)
You havn't check DNS error logs