The MAC address must be different on each device. The manufacturer should have seen to this.* Basically your thin clients do not conform to the standard. They are in error/broken.
You can do two things:
- Return them as broken.
Or at the least create a fuss at the manufacturer. This sort of thing should not happen and they need to be aware that they screwed up. - Try to work around it by changing the MAC after booting but before you bring the network up. (e.g.
ifconfig eth0 hw ether 00:11:22:33:44:55
somewhere in your boot scripts.
Your question is basically if you can do this: The answer to that is yes.
However if you do this then:
- Check all thin client for a sticker which proclaims which MAC they (should) use. Almost all network devices have such a sticker.
- Make sure you do not cause a conflict with another NIC.. If you already had a sticker with a unique value on them use that. Else carefully select some which are not already in use on your network. (And be prepared for unexpected conflicts if someone brings a new device to work which just happens to have the address which you just used).
- Document this.
- Do not use the MAC which those five client are using atm. Change all five. (Just to prevent surprised in the future. E.g. when a 6th device gets bought and added. Or when something fails and that device reverts to its illegal MAC).
* Having said that I have seen NICs in the wild which all had the same MAC.