Counting Duplicates - Displaying Unique Result

Hello,

In A1 and A2 - I have numbers like this:
100/10-05-009-10-W1
100/10-05-009-10-W1

The issue Im having is the first number is volume 1, the second number is volume 2...

Im trying to figure out a formula that will automatically look for duplicate numbers like listed above and when a duplicate number is found in the cell next to where the number appears (so B1, B2) the formula add's a 1 and 2

So for instance, if the numbers on the left were in column A, and the "1.0" and "2.0" were in column B, itd look like:

100/10-05-009-10-W1 1.0
100/10-05-009-10-W1 2.0


And if it only finds one instance of the number then it just puts a 1.0