For all these cases, I tend to insist just a bit more on the R when it’s doubled. This may vary between regionalisms however.
The difference can be heard when you make a break between the syllables:
- “Nous courons” is pronounced: Nous cou-rons
- “Nous courrons” is pronounced: Nous cour-rons – In this case the ‘r’ is emphasized because you pronounce it twice.
Definitely a nice question. Actually this distinction is only important with 3 verb roots in French (courir, mourir, quérir), in all other cases, one or two R don’t make a difference. But with courir, for example, there is a strict contrast between [ku.re] (PRS.2PL) and [kur.re] (FUT.2PL). This distinction even though appearing in very few contexts is contrastive for most native speakers.
Moreover, in the conditional present, arise two difficult configurations in the 1PL and 2PL whereby the two R are followed by a yod: courrions, courriez. These are the only words in French presenting such a sequence of phonemes /VrrjV/ where the two R have distinctive power. The form courrions cannot be pronounced [kurjõ], this is only appropriate for courions.
So speakers end up using a different structure avoiding these particular forms or treading uncharted territory and using one of the possible strategies: [kurərjõ], [kurirjõ] or making a special articulatory effort and producing [kurrjõ]. In all cases, feeling an certain amount of insecurity and self-conciousness about it.