In Quebec French, /a ~ ɑ/ and /ε ~ aε/ are phonemic, whereas in Hexagonal French, these vowels are merged to /a/ and /ε/, respectively. We tested the effects of extended exposure to Quebec French (QF) as a second dialect (D2) on Hexagonal French (HF) speakers’ abilities to perceive these contrasts. Three groups of listeners were recruited: (1) non-mobile HF speakers born and living in France (HF group); (2) non-mobile QF speakers born and living in Quebec (QF group); and mobile HF speakers having moved from France to Quebec (HF>QF group). To determine any fine-grained effects of second dialect (D2) exposure on the perception of vowel contrasts, participants completed a same–different discrimination task in which they listened to stimuli paired at different levels of acoustic similarity. As expected, QF listeners showed a significant advantage over the HF group in discriminating between /a ~ ɑ/ and /ε ~ aε/ pairs, thus suggesting an own-dialect advantage in perceptual discrimination. Interestingly, this own-dialect advantage appeared to be greater for the /ε ~ aε/ contrast. The QF listeners also showed an advantage over the HF>QF group, and, surprisingly, this advantage was greater than over the HF group. In other words, the results suggested that the acquisition of a second dialect did not enhance the abilities of listeners to perceive differences between phonemic contrasts in that D2. If anything, the acquisition of the D2 disadvantaged the perceptual abilities of the HF>QF group. This might be because these phonemes have, over time, become less acoustically marked for the HF>QF participants and have, potentially, become integrated into their D1 phonemic categories.