More than one validation rule in one cell

Hi all.
Is it possible to use both the following validation rules in the file attached:
1. A drop down list of the values "January" through "December" in columns A and B (done).
2. A warning message if the value in one of the cells within the range A11 through B10 is not unique.
for example "May" and "April" appears twice within that range of cells.
Thanks.