Hard to believe, but here are two English words that are essentially as thoroughly synonymous as "gorse" and furze."

Not very exciting though:

"inalienable"

"unalienable."

Semantically identical and orthographically nearly so - just one letter different out of eleven.

The only other difference, if it is a difference, is that the Declaration of Independence uses "unalienable," though most people (who'd be unlikely to use either one in a sentence) think it's "inalienable."

Oxford shows "inalienable" from 1647 and "unalienable" from 1611.