Reduplicated plural
A reduplicated plural is a grammatical form achieved by the superfluous use of a second plural ending.
Example
In English the plural is usually formed with the addition of 's': e.g. one cat, two cats; one chair, two chairs. In the Sussex dialect, however, until relatively recently there existed a reduplicated plural: e.g. one ghost, two ghostes/ghostesses; one post, two postes/postesses (note that here the Sussex pluralisation instead of adding just 's' after 'st', adds either 'es' as its usual plural, or a reduplicated 'esses'.[1] Kipling in Puck of Pook's Hill uses the Sussex reduplicated plural, employing for example 'pharisees' for fairies.[2] Tolkien in The Hobbit and The Lord of the Rings has Gollum speak with reduplicated plurals, often when complaining of "sneaky little hobbitses".
For the case when two different plural endings have historically been added onto a word (but one of them is no longer synchronically felt to have plural force, or even to be a distinct morpheme), see Double plural.