The most likely reason for the double reinforcement warning in slabs is that the slab boundaries are not created correctly. After bringing your floor to your screen so that you can see it completely in the plan window, click the slab analysis button under the Analysis menu. The red outer contours of your floors that are directly connected to each other at the same level are not created at once, if they are created piece by piece, your borders are incorrect.
Faulty slab boundaries and finite elements
In this case, you can check your tiles and find the faulty tiles and fix the situation, or you can delete all of them and try to place your tiles again. The contours of your properly placed tiles should be formed as shown in the screenshot below.
Correctly formed slab boundaries and finite elements