One possibility I encountered for that to happen is, if sometimes a modem/network router merges (automatically?) two entries from on device, eg. your laptop has Ethernet and a "WiFi" and the Router merges them to "Matt-H-computer".
That is just a name (mDNS, "zeroconf", "avahi" etc), has nothing to do with what goes on at ethernet level.
A more typical example for two devices to have same mac address is when they ship with a default "dummy" mac address from the factory, and need mac address to be set by firmware or software by the user. This is very common for NICs sold for "embedded" usage, for example the ENC28J60 module used with Plibox for Amiga, the NIC of the MiSTer systems etc.
However, sharing mac address can also be done on purpose, for other reasons - for example, link aggregation, where one host uses multiple NICs "as one" for redundancy and/or bundling. The standard protocol for this is called LACP, but various manufacturers also have proprietary variants.
A funny situation (which btw I suspect is the situation for Matt H) - is when two local LANs (VLANs) by accident are connected together, and hence appearing as one LAN on the hardware level, and every machine not only sees all NICs on the local LAN, but also via the ARP table of the router.
