tbiplẽ

Lyc. B tbiplẽ can be projected into Proto-Anatolian as *du̯i-plóm.

[D.S.]

The Greek, Italic, Celtic, and Lycian evidence converges semantically (‘double, twofold, twice’). The meaning of PGerm. *twīfla- ‘doubt’ can be explained by the well-documented semantic change from ‘twofold’ to ‘doubt’, cf., e.g., Gk. διπλόος ‘twofold; doubtful’, δοιή ‘doubt’ (Il.+) and Buck 1949a:1244f. The privative bahuvrīhi formation OAv. a‑bifra- ‘incomparable’ can be interpreted as ‘having no duplicate (*bifra‑)’, hence ‘existing only once’.
PIE *du̯i‑pló- means ‘twofold, twice’, cf. also Lyc. B trpplẽ ‘thrice’.

On the semantic history of the formation and the second compound member, cf. PIE *-pló- ‘-fold’.

Besides the zero‑grade formation PIE *du̯i‑pló‑ ‘twice’, which is attested as Lyc. B tbiplẽ ‘twice’, Gk. διπλός ‘twofold, double’, Lat. duplus ‘double’, and in OAv. a‑bifra- ‘incomparable’, also a full‑grade first compound member (*du̯ei̯‑) is reflected in OIr. díabul ‘double’ and PGerm. *twīfla- ‘doubt’.
While PGerm. *twīfla- ‘doubt’ can be explained as a substantivization of the adjective *du̯i‑pló- ‘twice’ with accent retraction and secondary full grade (cf., e.g., EDPG:530),LIPP II:168 n. 4 proposes analogy after the full grade of PIE *tréi̯- ‘3’.OIr. díabul ‘double’ (< *du̯ei̯‑) is originally an adjective.Note that PCelt. *du̯ē- is the standard compositional form of the numeral ‘2’ in Celtic (cf. OIr. dé‑riad ‘bigae’, W dwy‑flwydd ‘biennis’ etc., Thurneysen 1946a:242 §385.2, IEW:229); thus it is highly likely that OIr. díabul (< *du̯ē‑blo‑) is a replacement of PIE *du̯i‑pló- ‘twice’. Similarly, the Latin form du‑plus (not †bi‑plus) ‘twice’ as well as Umbr. dupla, tupler ‘two each’ are remodelled after the compositional form PItal. *du-, which is the standard form in Sabellic (at least in Umbrian, cf. Umbr. dat. pl. du‑pursus ‘biped’ [hapax, IgT VIb.10, Untermann 2000a:192]) and attested in Latin as well (cf. Lat. du‑pondium ‘coin worth two asses; measure of two feet’, IEW:229). Cf. also PAnat. *du̯o- ‘2’.However, the derivational bases of both formations substantiate the existence of an adjective PIE *du̯i‑pló- ‘twofold, twice’, cf. also the parallel formation PIE *tri‑pló- ‘threefold, thrice’.

For the underlying word-formational process and the analysis of the second compound member, cf. PIE *-pló- ‘-fold’.

[A.O.]

(?)OAv. nom. pl. n. a‑bifrā ‘incomparable’ (hapax, Y.33.13)The semantic approximation of Av. *bifra- as “Vergleich, Ähnlichkeit” by Pokorny (IEW:230) is misleading, because the semantic pathway from ‘two(‑fold)’ to ‘similar’ or ‘equal’ is unparalleled, cf. Buck 1949a:910–912. Instead, the only numeral meaning ‘equal; like, similar’ is ‘1’, cf. Lat. similis ‘like’ with sim- < PIE *sem- ‘1’. Cf. below, sub Semantic reconstruction.
Bartholomae 1904:92

Gk. διπλόος, διπλοῦς ‘twofold, double, twice, doubtful (Pi. N.10.89, Ph.1.302)’ (Hom.+)
Cret./Herac. adv. (loc.) διπλει, διπλη ‘twofold, twice’, δίπλ’ ἐρέω ‘I will say twice’ (Emped. fr. 17,1. 16 = Diels 1,117f.), διπλῇ ‘twice’ (Eur. Ion 760+)
GEW I:121f., GEW III:75, DELG:286, EDG:117, LSJ:436, Brugmann 1917a:128f.

PItal. *du‑plo‑, cf. Lat. duplus ‘double, twice’ (Plaut.+), Umbr. acc. pl. f. dupla (bis, IgT VIb.18), dat./abl. pl. m. tupler (hapax, IgT Va.19) ‘two each’
WH I:383f., EM:188, EDLIL:475, Untermann 2000a:191f., OLD²:638, Coleman 1992a:422f.

(late) OIr. adj./sbst. díabul ‘double, twofold; the double (quantity, repetition)’
EDPC:110, LEIA-D:65, eDIL s.v. 1 díabul, Greene 1992a:506

PGerm. *twīfla- ‘doubt’, cf. Goth. acc. sg. tweifl (hapax, Skeir.2.14), OHG zwîfal, etc.
EDPG:530, GED:351, Kluge & Seebold 2011a:1019, Ross & Berns 1992a:663