The Tamahaq of the Ahaggar Mountains attests a second sense of dub-ət that seems to be isolated even within Tuareg. Foucauld glosses it (p. 153) as:
2. ("by extension") 'be able to succeed someone (to an office), by virtue of his being your maternal uncle'
3. ("by extension") 'have as maternal uncle'
It yields the equally Ahaggar-specific word tădabit "person(s) of either sex with the right to succeed to someone's suzerainty due to the latter being their maternal uncle", used in the Ahaggar instead of pan-Tuareg tegăze. Examples include (retranscribed, perhaps imperfectly):
Biska d Mənnək ăddoben Musa daɣ ăra n tăññaten.
Biska and Mennek are potential successors to Musa by virtue of being sisters' children.
Luki d Mikela ăddoben Musa kaskab.
Luki and Mikela are potential successors in suzerainty to Musa.
Barka wa-n ăkli yăddobăt akli hin Mămmădu kaskab.
Barka the slave has as maternal uncle my slave Mămmădu, in a maternal uncle-nephew relationship.
Note the very un-Berber-looking word kaskab, lacking even the characteristic Berber nominal prefix, in the latter two examples. In the not obviously related sense of "metallic part of a camel bridle", akăskabbu (Tamasheq kiskab) is attested throughout Tuareg; but kaskab, in the relevant sense, appears just as unique to the Ahaggar as this sense of dub-ət. Foucauld's entry on the term runs to three pages (pp. 918-920), with neat kinship diagrams, but starts "in the direct line of succession to suzerainty, from maternal uncle to nephew or niece (in a kinship relation of maternal uncle to child of full sister or maternal sister (when speaking of succession to suzerainty over vassals))". One might be tempted to link the first half to Tuareg kus "inherit", but the vowel and the absence of any good explanation for the second half militates against it.
Not to beat around the bush, both dub-ət and kaskab look like great candidates for non-Berber substratum vocabulary loaned into Tuareg, especially in the kinship sense. Considered from this perspective, a non-Berber comparison for the former immediately presents itself: Songhay *túbí "inherit (v.); inheritance (n.)", with its derivative *túbá "sister's child (of either sex)" (the latter may be absent in Zarma and Dendi; both are absent from Northern Songhay, which substitutes Tuareg loanwords). Reflexes of the former include Zarma, Gorwol, Hombori, Djougou túbú (in Hombori also "succeed as chief", just as in Tuareg), Gao and Timbuktu tubu (in Gao also "bequeath, leave (to)"), Kikara túbí ...; of the latter, Gorwol túbéy, Gao tubey / tuba, Hombori túbê, Kikara túbá, Timbuktu tuba. (For modern Timbuktu Heath instead documents kaaya for "inherit", but Dupuis-Yacouba recorded "toubou".)
To my mind, a borrowing from Songhay into Tuareg looks more appealing, as I would expect a high-low tone if it came from Tuareg to reflect Tuareg stress; but the opposite direction could also be defended. Either way, however, there can be no reasonable doubt, given the good formal match and perfect semantic correspondence, that the Ahaggar forms are related to the Songhay ones. (Oddly enough, Nicolaï appears to have missed this connection in his wide-ranging hunt for Berber matches, instead focusing on Kabyle (originally Arabic) ətbəʕ "follow".) Yet their distribution is almost the opposite of what one would expect: in both groups, they are attested only in the varieties least in contact with the other. This suggests that the contact situation they reflect happened quite early, rather than being recent.
(References consulted include, for Tuareg, the dictionaries of Foucauld, Heath, and Alojaly; for Awjila, Paradisi; for cross-Berber comparison, Naït-Zerrad; for Songhay, Heath, White-Kaba, Ducroz and Charles, Zima, and Dupuis-Yacouba, not to mention Nicolaï's La force des choses.)