There is nothing called mount control file,actually it is mounting database.When u mount the database oracle will look for control file.If it cannot find in the specified path given in the parameter file it will give this error.So check wheather the control file is present or not in the specified path.
from oracle documentation
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ORA-00236 snapshot operation disallowed: mounted controlfile is a backup
Cause: Attempting to invoke cfileSetSnapshotName, cfileMakeAndUseSnapshot, or cfileUseSnapshot when the currently mounted control file is a backup control file.
Action: Mount a current control file and retry the operation.
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as for your suggestion, the control file is already there.
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