doubtlessly

From Middle English doutelesly, dowtelesly, equivalent to doubtless +‎ -ly.

doubtlessly (comparative more doubtlessly, superlative most doubtlessly)

1. (sometimes proscribed) Without question; indubitably.

   There are doubtlessly more insects than mammals.

2. (obsolete) Without feeling doubt, without misgivings.
   • c. 1587–1588, [Christopher Marlowe], Tamburlaine the Great. […] The First Part […], 2nd edition, part 1, London: […] [R. Robinson for] Richard Iones, […], published 1592, →OCLC; reprinted as Tamburlaine the Great (A Scolar Press Facsimile), Menston, Yorkshire, London: Scolar Press, 1973, →ISBN, Act II, scene vi:

     Some powers diuine, or els infernall, mixt
     Their angry ſeedes at his conception:
     For he was neuer ſprong of humaine race,
     Since with the ſpirit of his fearefull pride,
     He dares so doubtleſly reſolue of rule.

Sometimes considered a redundancy, as doubtless (without the -ly suffix) functions as an adverb with an identical meaning. The synonyms indubitably, undoubtedly, and unquestionably all require the -ly suffix.