Some older - but widely used - version of the unity gaming engine has a bug that leads to some games registering keypresses twice.
I encountered this problem repeatedly now and always forgot the workaround as soon as I implemented it. This blogpost is a note to my future self, so I know where I can find this information, once I will need it again.
Within Steam click on the game entry and then on “Properties”. On tab “general” click on the “start options” button. Paste the following in the input field:
This solves the problem.