Prevent Lightroom importing JPG JPEG

In your Lightroom preferences general tab, make sure "Treat JPEG files next to raw files as seperate photos" is not selected.

Most cameras that shoot Raw offer the option to shoot Raw+JPG, or Raw only - but not all. My little LX1 is one that does not give the option, so I get this same issue.

If you are using some other method to get the image files into your computer first, and then using the Add option to put them into LR, then you can just leave out these JPGs at that earlier point. However, IMO it is more convenient to have LR do the importing as a single operation. The idea of manually deleting the JPGs from the camera card before safely copying the pictures into the PC, scares me too much to contemplate doing that.

LR does not offer any simple built-in solution here, except by selecting the option to show Raw and JPG as separate images instead of pairing them up (which is what people sometimes do actively want). Then the JPG can be either deleted after import, or else de-selected in the Import dialog and not included in the first place.

If you delete the unwanted JPG, outside LR, after import, LR will not complain - but may continue to (informationally) report the image concerned as "Raw+JPG", even though that is no longer the case physically.

I don't have LR in front of me now, but I am pretty sure there is a sort order option for the Import screen. So if you can separate the images on your camera card by their filetype, and if you know that there are no JPGs on that camera card that you want to import into LR, you can easily highlight the Raw files as a group - and then proceed to Import just those into the computer.

Or: if you are interested in shooting Raw only, you can set your camera JPG settings to the fewest available JPG megapixels and the lowest possible saving quality. This makes a dramatic difference to the JPG file size, and they may well then be accepted as doing no real harm (not treating them as separate images in that case, just as non-functional sidecar files).