Maybe this is not such a rare occurence, but definietly a first for me!
If not obvious from the picture: the end of the fruit truss continued to grow as a sucker producing new leaves and some flowers. Pretty unusual sight.
It sometimes happens with my tomatoes too.
What's the variety, Andras? Indeterminates do it. Unusual for determinates.
Pretty normal, I would say. Just snap it off at the node.
Are you removing all the side-shoots, when they are small?
..happens on mine all the time... that is, if you are a bit lax in noticing it happening in the first place, as usually I would snap that off before it got very far...