In my dialect/idiolect "roar" /rɔ˞/ isn't long, but then there's the issue about the rhotic vowels vs a properly consonantal sound. I also wouldn't describe "rare" /rer/ as being long (though it is tense). I probably would call "rear" /rir/ long, though I don't know whether length is really the important factor (or if it's just an effect of the vowel quality). But yeah, if the diphthongs matter, it's definitely because they give enough "space" between the identical liquids. I could even imagine a lengthening process that converts monophthongs into diphthongs in this context (assuming diphthongs matter in the first place).

I just thought of another one: "drawer" which in my dialect is the single syllable /drɔ˞/ (becoming [ʤrɔ˞] via standard assimilation of TR clusters).