How do I locate 2 similar numbers in a column?

Example:

Cells a1 to a10 contains numbers 1-10 and cell a11 contains the number 5.

I want to locate the number 5, by putting a formula in the adjacent column so that cell b11 announces "redundant". Is it a Lookup or what?

To put it in a query in Column B: If number x (cell) is redundant in column A, write "redundant".

Any helpfull suggestions?

TIA