If we keep two magnets with same poles together, then they'll become weaker overtime. I once deliberately did it to find out what would happen.
This could be explained with Magnetic Domain Theory. In magnets, the magnetic domains are aligned in the same direction, giving them a strong magnetic field. If two magnets are kept with like poles facing each other,
... the effect will be similar to applying an external magnetic field to the magnet in direction opposite to the magnetic field of the magnet. This will result in the magnetic domains trying to align with the external magnetic field, and in this case, as the magnetic fields of both magnets are equal, some of the domains will disorient, thus weakening the magnetic field of each magnet,