Peugeot 207 starter constantly engages

My 207cc has an intermittent fault where the starter will energise while the engine is running.
I have replaced two starter motors in the last 6 months because unless you are not actually driving you don’t know it’s happening.
I know it happens as I got to work, turned off the ignition and took the key out and it was trying to turn it over so I removed the pos from the battery.
I went back to it later in the day and the fault had cleared itself but it has come back again.
This time the wife was driving and didn’t realise what was happening and it drained the battery whilst she was driving. Every time I go to investigate the fault isn’t there. It so frustrating.
Has anyone come across this or fixed it? Any help welcome please.