Hebrew likes to repeat words. You will need to check grammars and lexicons for the particular meaning. In this case, it has the sense of all generations:
(iv) When the same noun is repeated in the singular—with or without the conjunction וְ or with a preposition—it has a distributive sense. (Cf. §29.3/(viii).)
year by year (Deut. 14:22)
all generations / every generation (Deut. 32:7)
year by year (Deut. 15:20)
morning by morning (Exod. 16:21)
Van der Merwe, C., Naudé, J., Kroeze, J., Van der Merwe, C., Naudé, J., & Kroeze, J. (1999). A Biblical Hebrew Reference Grammar (electronic ed., p. 184). Sheffield: Sheffield Academic Press.
Another reference is BDB:
b. usually of duration to come, future age(s), לְדֹר דֹּר Ex 3:15
Brown, F., Driver, S. R., & Briggs, C. A. (1977). Enhanced Brown-Driver-Briggs Hebrew and English Lexicon (p. 189). Oxford: Clarendon Press.
Here it is in Gesenius's grammar:
(d) The repetition of single words, and even of whole groups of words, especially to express entirety, or in a distributive sense. The following cases are more particularly to be noticed:
1. The repetition of one or more words to express the idea of every, all, as יוֹם יוֹם Gn 39:10, &c., day by day, every day; שָׁנָה שָׁנָה year by year, Dt 14:22; אִישׁ אִישׁ every man, Ex 36:4; with בְּ before each, as בַּבֹּ֫קֶר בַּבֹּ֫קֶר Ex 16:21 every morning (and similarly before a group of words, Lv 24:8), for which the distributive לְ is also used, לַבֹּ֫קֶר לַבֹּ֫קֶר 1 Ch 9:27, and with one plural לַבְּקָרִים Ps 73:14, Jb 7:18 parallel with לִרְגָעִים every moment. Somewhat different are the instances with בְּ before the second word only, e.g. יוֹם בְּיוֹם day by day, 1 Ch 12:22; שָׁנָה בְשָׁנָה year by year, Dt 15:20, 1 S 1:7 (but in verse 3 מִיָּמִים יָמִ֫ימָה), כְּפַ֫עַם בְּפַ֫עַם Nu 24:1, Ju 16:20, 20:30 f., 1 S 3:10 as at other times. Also With the two words united by means of wāw copulative, אִישׁ וְאִישׁ Ps 87:5, or אִישׁ וָאִישׁ Est 1:8; דּוֹר וָדוֹר all generations, Dt 32:7; יוֹם וָיוֹם Est 3:4; cf. Est 8:9, Ezr 10:14, 1 Ch 26:13 and often (cf. Cheyne, Bampton Lectures, 1889, p. 479, according to whom the use of the ו copulative with the second word is especially common in Ch and Est, and therefore belongs to the later language; Driver, Introd.6, p. 538, No. 35); sometimes (but with the exception of Ps 45:18 only in very late passages) with a pleonastic כָּל־ preceding, Ps 145:13, Est 2:11, 9:28, 2 Ch 11:12, &c.
Gesenius, F. W. (1910). Gesenius’ Hebrew grammar. (E. Kautzsch & S. A. E. Cowley, Eds.) (2d English ed., pp. 395–396). Oxford: Clarendon Press.