A Sindarin verb for “return, go back” as a combination of √(N)DAN “back” and √MEN “go”, contrasted with dandol- “return, come back” appearing in notes from around 1959 (PE17/166). Tolkien first gave this verb as {namen >>} damen reflecting vacillation on the primitive prefix *nan- > *ndan-. Later in the same notes he gave a form dadwen, reflecting vacillation of the development of ancient nm in Sindarin, to either mm or dw.

Neo-Sindarin: For purposes of Neo-Sindarin, I would use ᴺS. dammen- for this word as the most phonologically plausible. Tolkien represented intervocalic m as both mm and m in Sindarin (e.g. lammad vs. minlamad), so damen- is possible as well. Either way, the pronunciation would be [with damen] (short [m]).