If you’ve ever said libry instead of library, or Febry instead of February, then you have perpetrated haplology, the omission of one of a pair of sounds or syllables.
The word was invented by the American philologist Maurice Bloomfield at the end of the nineteenth century. He derived it from the Greek haplos, one or single, and –logy, a word or speech. It’s very common in English speech to drop the second of a pair of repeated sounds like this. A nice irony is that haplology is just the sort of word to which haplology happens ...
It’s a special case of what’s called syncopation, a grammatical term for losing any kind of sound in the middle of a word, such as the poetic shortening of even to e’en, or the way pacifist has been created from the longer pacificist that was its original spelling.
In writing, the equivalent is the rarer haplography — making the mistake of writing philogy instead of philology for example.