Removing each row that possesses a singleton in the first column

I would like to remove each row that possesses a singleton in the first column. e.g. Input: Letters represent variables of which there may be thousands. Numbers represent columns and rows. There are multiple columns (not shown here). 1. 1. A 2. B 3. C 4. C 5. D 6. E 7. A 8. H 9. D 10. D Output: 1. 1. A 3. C 4. C 5. D 7. A 9. D 10. D