Excel 2007 : Indentify duplicate values with a condition

Hello Everyone,

I'm having some trouble creating a formula in identifying duplicate values.

Don't know if a array would do the trick...this might be something simple but result would have to be as depicted below.

Question:
How can I identify unique values and define them under column B1 results:

Values under column A1:
123
123
123
146
146
150

Results under column B1 should be
123-1
123-2
123-3
146-1
146-2
150-1

I appreciate any assistance given.

Thank you,
Antonio