The rules say this about contested actions:
Whoever rolls the highest succeeds at the action.
What happens if two or more characters roll the same? Do they roll again for a tie? Is the attribute score tie breaker, like with initiative, and in case of tie there, the winner is determined randomly? Does the GM think of a suitable outcome?
Is there an answer in the rules or is this ruled by the GM on a case by case basis?
Up to the GM
No way to answer you unless you give a specific situation, and then it can still vary. What makes sense for the scene/narrative?
What kind of contested action is it? How important is it? How invested are the players and/or characters in it?
Some cases you might just declare a tie and then have them go again. Some cases they both take damage. Some cases it causes them to lose favor with the crowd. Some cases whoever has the higher attribute score makes sense.
Just do what makes the most sense for the moment and what is actually happening.
I was asking in general, because the rules didn’t mention the possibility that there may be a tie. Is it possible to add something in the lines of your comment to the official rules? 
It is addressed somewhat in the rules if you look in the Combat Chapter under Initiative:
In case of ties, characters act in order of their Agility scores (from high to low). If Agility scores are also tied, determine the order randomly.
Right now not any plans to modify the rules until things have been sorted with all the kickstarter stuff from what I understand.
Something that might be added to the wiki though.