5 answers
Ok, try this:
Create 2 field updates for your workflow rule:
1. IF(BEGINS(Field, 'ABC'), '', 'ABC ') &
SUBSTITUTE(Field, ',', ', ABC')
2. SUBSTITUTE(Field, 'ABC ABC', 'ABC')
The first will replace all commas with comma and your text
The second will edit remove any duplicates if the text is already there.
The only thing to be careful of is SUBSTITUTE is case sensitive so it will not cover is if someone enters Abc or abc or AbC.