Why do my herbs keep getting flies

[SwedishHills]

I've been potting herbs bought from the supermarket in plant pots to keep on my windowsill but twice now the basil and coriander have been getting lots of little flies in them so I have to throw them 😢

This has happened twice now.. any idea how I can stop this? They appear after about 5 days and once I get rid of the herbs no flies anywhere so they aren't in the house already..

Get a fly eating plant and put it next to the herbs - I read this in the RHS Magazine and it really works! Lot of different fly eating plants are available, doesn't have to be a venus flytrap, I have 2 lovely little plants with sticky leaves and v pretty delicate pink flowers.

Fungus gnats. Top the compost with gravel and water as little as possible (or from underneath). I think that's supposed to work but google them for more ideas.

Sand on top of the compost. They're coming from there. The larva munch on the roots.

Water from the bottom not the top.

Also shop bought herbs are massively over crowded in the pots, and any plant not growing in ideal conditions will always be more susceptable to pests.