The Hebrew word from which we derive our English word Jehovah has been called the Tetragrammaton, which is often shortened to JHWH or YHWH, the English equivalent of the four Hebrew letters (viz., Hei Vav Hei Yod, or Yud, Hay, Vav, Hay. ) which together are pronounced yä′wā, or yä′wĕ.
Occurring over 6800 times in the Tanakh (Old Testament), it not only the most common name for God in the Tanakh, but it is also God's "personal" name, the name God gave to Moses to give to the Israelites if they were to ask Moses, "Who is this God who sent you to us?" Here's the context:
Moses said to God, “If I go to the Israelites and tell them, ‘The God of your fathers has sent me to you,’ and they ask me, ‘What is his name?’ – what should I say to them?” God said to Moses, “I AM that I AM.” And he said, “You must say this to the Israelites, ‘I AM has sent me to you.’”
YHWH as the name for God was not news to Moses or to the people he led out of captivity. Perhaps, as Constable observed, the Israelites needed to know if the God who appeared to Moses and sent him to them was the same God they had been worshiping for centuries. His name was YHWH (or Jehovah, or LORD--all capital letters).
Heb “Yahweh,” traditionally rendered “the LORD.” First the verb “I AM” was used (v. 14) in place of the name to indicate its meaning and to remind Moses of God’s promise to be with him (v. 12). Now in v. 15 the actual name is used for clear identification: “Yahweh…has sent me.” This is the name that the patriarchs invoked and proclaimed in the land of Canaan [my emphasis].
God's personal name is used twice in the verse you cite:
The LORD is a warrior, the LORD is his name.
As for why the word LORD (YHWH) appears twice in the text, Exodus 15:3 is part of a song the Israelites sang after they had miraculously crossed the Red Sea (or Sea of Reeds) and in doing so had been delivered from their former Egyptian captors, who were drowned in the same sea.
[Parenthetically, the Israelites sang the right song, but on the wrong side. In other words, had they begun singing the song--with slightly different words, of course--prior to stepping into the Red Sea, they would have testified to God's ability to save them before He saved them. How much easier for believers in God to sing songs of praise but only after He has saved them, rather than before He saves them. Singing His praises before the miracle requires a strong and steadfast faith]
As part of a song, the verse in question conforms to Hebrew poetry, the most obvious characteristic of which is parallelism. The type of parallelism in verse 15 is synonymous, which means simply that a similar idea is expressed in two slightly different--but synonymous--ways. The technical name for the two parts is a bicolon or a distich.
The pattern of parallelism is far from "random" (as you put it) but is part and parcel of God's eternal Word, particularly in songs, as found in Exodus 15 and throughout the book of Psalms. Moreover, according to this particular distich, not only is Jehovah a warrior, but He is known by the name Jehovah, which to the Israelites meant, in part,
- the eternally existent one
- the person who simply IS,
- the one whose ground of being is His eternal self
- the covenant-making, covenant-keeping God
- the one true God (see Deuteronomy 6:4-9, a prayer--the Shema--which says in part, "Hear O Israel, the LORD our God is One LORD")
In more modern parlance we might say today that God is the unmoved mover, the un-caused cause.
In conclusion, part of the significance of Exodus 15:3 is that it gives insight into the character of Jehovah. Not only is he a conquering warrior who can rout any and all enemies of his people, but he is also an eternal warrior who until time is no more will not only do battle with the forces of evil but will also ultimately defeat them and will be hailed forever thereafter as the conquering warrior/hero.
And what is the last enemy of humankind which God will ultimately defeat? Death itself. As the apostle Paul put it,
The last enemy to be destroyed is death (1 Corinthians 15:26 NIV).
And as the apostle John put it,
Then death and Hades were thrown into the lake of fire. This is the second death, the lake of fire. And if anyone’s name was not found written in the book of life, he was thrown into the lake of fire (Revelation 20:14-15 NASB).