Many American speakers drop one /r/ from words which have two /r/s. The deleted segment is always post-vocalic. This type of deletion is normally produced by rhotic speakers who tend to retain the /r/ in every other position. It is a dissimilation process because it avoids the presence of identical segments in the same word.
The process usually occurs with adjacent syllables such as in larger but also with intervening syllables such as in thermometer. The latter is an example of long-distance assimilation. Deletion usually occurs in unstressed syllables, but can also occur in stressed syllables such as in farther.
Here is a list of words with r-dissimlation:
(The /r/ in parentheses indicates the deleted segment).
The deleted /r/ is usually the first post-vocalic /r/ in the word, but this is not the case in formerly, forward and northerner. Notice that in these cases the deleted /r/ is unstressed. R-dissimilation doesn't apply to word-final position. Here the /r/ is always maintained.
R-dissimilation appears to be most common between labial and coronal consonants. This is the case in adversary, caterpillar, farther, formerly, governor, northerner, particular, repertoire, southerner, surprise, thermometer, vernacular and westerner. It doesn't occur with velar consonants. For example, the first /r/ is never deleted in worker.
R-dissimilation is common in many varieties of American English. This phonological process deletes a post-vocalic /r/ from a word with two. The deleted /r/ is usually unstressed and between coronal and labial consonants. The speakers who apply r-dissimilation usually retain the /r/ in all other positions.