Akkadian

dāru - long time, eternity (AHw. 164, CAD D 107)

See also dūru ‘eternity; permanent position’ (AHw. 178, CAD D 197), dūriš ‘forever’ (AHw. 178), dāriš ‘forever’ (AHw. 163, CAD D 113), dāriš ‘for eternity’ (AHw. 164, CAD D 114), dārānu ‘eternal’/’eternal sacrifices’ (AYw. 163, CAD D 109), dārīˀu ‘(ever)lasting, eternal’ (AHw. 164, CAD D 115), dārīˀa ‘eternally’ (CAD D 112), dārītu (AHw. 164, CAD D 114), dārūtu (AHw. 164, CAD D 118), dārūtaš (AHw. 1550, CAD D 118). This derivation, unquestionable in terms of the underlying consonantal root and the semantic development, is not without problems as far as derivational morphology is concerned, as the expected prototype *dawr- (Marrassini 1971: 48, Fronzaroli 1965: 143, 148) would yield dūru in Akkadian, actually well attested with the same meaning (see above). No ready solution for this problem is at hand. Shall one postulate a bivocalic pattern *dawar- contracted into *dār- in the prehistory of Akkadian? Derivation from the by-form *dahr- is not to be ruled out completely, but does not seem likely.

PS
*dawr-/*dawar- - period, cycle (DRS 240, Marassini 1971: 48, Fronzaroli 1965: 143, 148, EDA II)

The – apparently iconic (descriptive) – reduplication *dār- dār- (or *dār- dawr-), attested in both Akkadian and early NWS (Ugaritic, Hebrew, Aramaic), may go back to PS.