מוּת muth (H4191) means "to die" and comes from a root word that literally means "flowing or chaos end" and is concretely represented as a man. The first time is used in Torah is in Genesis 2:17, where the word is used twice at the end of the verse: "מוֹת תָּמוּת mot tamut" which literally translates to, "die you died" because the prefix תָּ (tav with kamatz vowel point) in Hebrew is a second person masculine singular suffix that means "you did". The Greek Septuagint uses θανάτῳ ἀποθανεῖσθε thanáto apothaneísthe (G2288 and G599) which literally translates to, "you will die by death." Thus, the Septuagint faithfully represents the original Hebrew unlike the vast majority of Bible translations which translate to "מוֹת תָּמוּת mot tamut" to "you will surely die".
It's common among translators of Biblical Hebrew to assume that repeating a Hebrew word twice in a row can be used to emphasize a concept, like saying "מְאֹד מְאֹד maod maod" in Genesis 7:19 (very very) to mean "very or exceedingly much" or "אָמֵן אָמֵן amen amen" in Numbers 5:22 or John 14:12 sometimes translates to "Most certainly I tell you..." However, the word מוֹת mot in Genesis 1:27 is not actually used twice, but has a variation of the word as תָּמוּת tamut which means "you died".
מוּת muth may be the origin of the English word "mute" (not uttering words, or not having the power of utterance) and "moot" (an irrelevant, hypothetical case used for practice by law students, i.e. "moot point")

cf. Exodus 21:15, 17; Leviticus 20:9