Two photos, different formats ??

HEIC is a format adopted by Apple a couple of years ago which offers a higher level of compression than JPG whilst maintaining quality. You only really need to keep one version of the photo.

Before you delete one of the versions, its worth double checking that the resolution of both photos is the same by highlighting photo and pressing cmd-I

As to why you are seeing them both - different import methods my import the different formats. Eg my photostream, direct USB connection, iCloud etc.

Make sure you are consisitent with how you transfer photos.

I don't think there are any downsides. When you export from Photos you can choose which format to use.

When I share pictures from Photos and send to an Outlook account they seem to automatically convert to jpg.

If it ever becomes a problem you can change the setting on your phone to 'Most compatible' which will then save your photos in jpg rather than HEIC