The only free and comprehensive online etymological dictionary of the Spanish language
dulce m. (Adjective, Noun) "sweet;" "candy"

10th cent. Old Spanish duz, duce. From Latin dulcis "sweet." From Proto-Italic *dulkwi- 'id.' Of unknown origin, presumably borrowed from another language.

Romance: Western Vulgar Latin: Asturian dulce, Portuguese doce, Galician doce, Catalan dolç, French doux, Italian dolce; Eastern Vulgar Latin: Aromanian dultsi, Romanian dulce; Sardinian: durke

Indo-European: Hellenic: Ancient Greek γλυκύς (glukys), Mycenaean de-re-u-ko

The adjective "sweet" is original. The noun "candy" is a later innovation, though the innovation may have occurred in very ancient times (the secondary meaning of "dessert" or "candy" spans the Western Romance languages).
dura f. (Noun) "duration"

From Latin durus "hard" (see duro). The meaning broadened in Latin from "hard" to "lasting over a period of time."

Romance: Western Vulgar Latin: Portuguese duro, Galician duro, Catalan dur, French dur, Italian duro; Eastern Vulgar Latin: Romanian dur; Sardinian: duru
durante (Adverb) "during"

15th cent. Originally meaning "hard." From Latin durans "enduring" (but originally "hardening;" contrast dura and duro), a past participle of durare "to harden," itself from durus "hard" (see duro).
duro (Adjective) "hard"

13th cent. From Latin durus 'id.' From Proto-Italic *dūro- 'id.' From Proto-Indo-European *du̯h2-ro- 'id.' From the root *du̯eh2- "to endure."

Also the origin of the surname Duro and its diminutive surname Durelo.

Romance: Western Vulgar Latin: Portuguese duro, Galician duro, Catalan dur, French dur, Italian duro; Eastern Vulgar Latin: Romanian dur; Sardinian: duru

Indo-European: Hellenic: Ancient Greek δηρός (derós); Armenian: erkar; Indo-Iranian: Sanskrit dūrá- "far," Avestan dūrāt̃ 'id.'