Where is the Queen placed when a C/M is already at center?
According to the Laws of Carrom 94


  If the Centre Circle is partially or completely covered by other C/M, the Queen shall be placed so as to occupy most of the uncovered portion or in any position adjacent to the Centre Circle in such a way so that it is not easy to pocket for the player having his turn. The placing of the Queen by the Umpire, in such a situation, shall be final.