bash script search file and insert character when match found

Hi I need a bash script that can search through a text file and when it finds 'FSS1206' I need to put a Letter F 100 spaces after the second instance of FSS1206

The format is the same throughout the file I need to repeat this on every time it finds the second 'FSS1206' in the file

I have really not explained this very well but see example below, I need an F at that position. :wall:

or would it be easier to search for "71502FSS1206 FSS1206" and put a F 100 spaces after this rather than just search for a single instance of FSS1206?

71502FSS1206               FSS1206               000000000000200100000000000820009958170009958181200100005952
71502FSS1206               FSS1206               000000000000200100000000000820009958170009958181200100005952                       F