The Sindarin word for “double” in notes from the late 1960s derived from primitive ✶atatya, an adjective form of the root √ATATA (VT42/26-27).
Conceptual Development: The Etymologies of the 1930s had N. tadol “double” under the root ᴹ√TATA “two”, an adjectival form of tâd “two” (Ety/TATA). This word reappeared in its mutated form dadol in notes on The Feanorian Alphabet from the 1930s and 40s in the term peg dadol “double dot” (PE22/31; PE23/22).
A (Sindarin?) word for “double” in the name Adurant “Double Stream” from The Silmarillion (S/123; SA/rant).
Conceptual Development: In The Etymologies of the 1930s, Adurant was Ilkorin and its initial element was Ilk. adu/ado “double”, cognate to ᴹQ. atwa of the same meaning and derived from the root ᴹ√AT(AT) “back, again” (Ety/AT(AT)).
Neo-Sindarin: For purposes of Neo-Sindarin, I would use adu only as a prefix, and would use edaid as the normal word for “double”. This prefixal use of adu- “double” would help distinguish it from ad- “re-, again”.