Hi Hubertus,
When the user links two objects via a UI form, the system makes sure there are no duplicate links - even if the user tries to link the same objects twice. When you use INSERT action from a process, however, the system cannot detect duplicate links.
The solution is to add a condition to the INSERT rule in your process to check that the link does not already exist. Alternatively, instead of using INSERT action you can use assignment action:
Order.Tour = Tour
This should not cause problems even if executed multiple times.
