As I’ve mentioned before, a few Russian nouns have a plural form bearing no resemblance to their singular form. One example is человек (/čelovek/ ‘person’), which has the plural люди (/ljudi/ ‘people’). (The form čelovek is sometimes possible in the Genitive Plural.)
When different inflected forms of a word (for example, a noun) come from different roots, the relationship between those roots is called suppletion. Surprising noun plurals in Russian – Language Miscellany
Another Slavonic language takes the suppletive pair /čelovek/—/ljudi/ a step further. Unlike Russian and almost all other modern Slavonic languages, Slovene retains a numerical category ‘dual’, separate from both singular and plural. In Slovene:
- the singular forms are based on the stem človek-
- the plural forms are based on the stem ljud-
- the dual forms are based on ljud- in the genitive and locative cases, but on človek-in the other cases (nominative, accusative, dative and instrumental). So, for the dual, there is suppletion between the genitive/locative form and the form for the other 4 cases.
In fact, the genitive dual ljudi and locative dual form ljudeh are completely identical with their plural counterparts. This is one example of wider pattern in which most dual inflections for genitive and locative are identical with the corresponding plurals.
Source
The Syntax-Morphology Interface: A Study of Syncretism, Matthew Baerman, Dunstan Brown, Greville G Corbett (2005)