When you have redundant constraints in your model, the computed forces in your model can not be guaranteed to be correct. While solving the system, Simulink needs to disregard one of the constrains in order to make the simulation succeed. Often redundant constraints can lead to errors while simulating so it is a good practice to avoid them.
An example is attached which shows the difference between a correctly modeled set of constraints and a model which contains redundant constraints. Both models represent a portal which is supported at both ends. In the incorrect model this has been done using two weld joints. As one weld joint does already fix the complete movement of the portal, the second weld is completely redundant. In the sensor readouts you can see the weight of the portal is not distributed evenly over its legs, which would be what you expect. In the correct model a rotation is possible around one of the legs, which is fixed by introducing a constraint on the y-movement of the other leg. This way there are no redundant constraints and you will see a correct load distribution on the sensor readout.
