Fing is a good answer, and any software that will do an arp scan (who-has) also.
That will help you identify what though not why,
The most common causes are
(1) manual IP addressing alongside DHCP or
(2) more than one DHCP server, with address pools that overlap
with (1) typically the manual IP addresses are not noted down anywhere or are lost track of
the answer to this is DHCP reservation, which is a table of each mac address with its fixed IP that you enter at the DHCP server
you should do this even for those items that are currently manually configured, as this will tell the DHCP server not to give them to anything else, but also lets you change those devices to automatic addressing later
with (2) typically it is a where someone has tried to repurpose a spare home router as a switch to provide more ports,
and forgot to turn DHCP off (or didn't know they needed to) - if so do that or replace with a cheap unmanaged switch unless you need something more complicated.
Fing or other may tell you if more than one device is listening on port 67/udp (DHCP).
Some computers will tell you which DHCP server gave out their address (ipconfig /all in Windows?)
prompt $P - Invalid drive specification - Abort, Retry, Fail? $G
prlzx on iDNET: ADSL2+ / 21CN at ~4Mbps / 700kbps with IP4/6