If it's a routine thing, and you did give a fluke dose a couple of weeks ago, then you could leave it maybe 6 weeks in all before giving another fluke dose. But as shep53 says, check the dosage info on the wormer, some are only effective against fluke at a much higher dose rate.
If you have concerns this fella may have fluke then I'd dose sooner rather than later - as it's a couple of weeks since his last meds, you could dose now with a different med.