Manni is an Old Danish masculine name originally developed as a byname from man ‘man’; there is occasional confusion with the name Magni (von Feilitzen 1937: 324; Fellows Jensen 1968: 194-5).
There are only three entries in the PASE corpus for Manni, of which Manni 1 was abbot of Evesham between 1044 and 1058, Manni 2 represents one or more people occurring as witnesses in three charters between 1044 and c.1053 (on which see Manni 4 below), and Manni 3 is a catch-all for occurrences in DB; it is also possible that some of the moneyers recorded under Manning in PASE actually represent Manni. Even so, it is clear that the name was extremely rare in England.
Fellows Jensen 1968: Gillian Fellows Jensen, Scandinavian Personal Names in Lincolnshire and Yorkshire (Copenhagen: I Kommission hos Akademisk Forlag, 1968)
von Feilitzen 1937: Olof von Feilitzen, The Pre-Conquest Personal Names of Domesday Book, Nomina Germanica 3 (Uppsala: Almqvist and Wiksells, 1937)
Spellings in Domesday Book: Magno, Manni(us), Mannig, Mannio, Mannis, Manno
Spellings in Exon: Mannius
Spellings in FBB: Manne
Forms in modern scholarship:
von Feilitzen head forms: Magni, Manni
Phillimore edition: Magnus, Manni
Alecto edition: Magni, Manni