haplology
Linguistics term for the omission of one of two consecutive identical or similar syllables, e.g. 図書館 (としょかん) from 図書 (としょ) + 館 (かん) instead of としょかかん.
"Haplology" is a technical term used in linguistics.
"Toshokan" (library) comes from "tosho" (books) and "kan" (building), but through haplology it became "toshokan" rather than "toshokakan".