would help if I used the code tags gees sorry about that

ID           SEQ
1              1
1              2
3              1
2              1
1              3
2              2
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This is what I want it to output. The problem is the ID aren't consecutive in my file. The question is how to write this so it looks at the ID and KNOWS 1) if it's the first time it's come up and 2) if it's not the first time what to set the SEQ number to?