This year's sales were double last year's.
This box is twice as big as that one.
二倍 explicitly spells out 'two times' by attaching the number, while 倍 alone can already stand for 'double' as a bare noun.
一倍 now usually means the same amount (×1), while 二倍 means double (×2), even though 一倍 historically also meant 'double', causing confusion.
二倍 is a noun meaning 'twice the amount' as a static quantity, whereas 倍増 describes the process or event of doubling.
Standard kanji spelling.
Common in informal writing, advertisements, or when numerals are preferred.
Compound of 二 (two) and 倍 (times, -fold).