Free variation

Free variation in linguistics is the phenomenon of two (or more) sounds or forms appearing in the same environment without a change in meaning and without being considered incorrect by native speakers.

Examples from English include:

When phonemes are in free variation, speakers are sometimes strongly aware of the fact (especially where such variation is only visible across a dialectal or sociolectal divide), and will note, for example, that tomato is pronounced differently in British and American English, or that either has two pronunciations which are fairly randomly distributed. However, only a very small proportion of English words show such variations. In the case of allophones, however, free variation is exceedingly common, and, along with differing intonation patterns, variation in allophony is the most important single feature in the characterization of regional accents.

...
Wikipedia