The original can be translated as either "Yahweh" or "Jehovah". But taking their self-assured attitude in using this text, here are a couple of answers you can give back:

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Isaiah 42:8 (ASV) "I am Jehovah, that is my name; and my glory will I not give to another, neither my praise unto graven images."
Compare that with:

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Hebrews 1:6-8 (ASV) "And when he again bringeth in the firstborn into the world he saith, And let all the angels of God worship him. 7 And of the angels he saith, Who maketh his angels winds, And his ministers a flame a fire: 8 but of the Son he saith, Thy throne, O God, is for ever and ever; And the sceptre of uprightness is the sceptre of thy kingdom."
Not only does He (Jehovah) command the angels to worship Him (the LORD Christ), which unless Jesus was Jehovah God would be in direct contradiction to Jehovah's own prohibition in the Isa 42:8 passage, but in Heb 1:8, Jehovah Himself calls Jesus, "GOD" (Jehovah).

Secondly, GOD has many names by which He has revealed Himself. Each name highlights part of His character and/or being. This is how Jehovah God spoke to Moses in regard to His name:

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Exodus 3:13-14 (ASV) "And Moses said unto God, Behold, when I come unto the children of Israel, and shall say unto them, The God of your fathers hath sent me unto you; and they shall say to me, What is his name? What shall I say unto them? 14 And God said unto Moses, I AM THAT I AM: and he said, Thus shalt thou say unto the children of Israel, I AM hath sent me unto you."
Here, GOD tells Moses that His name is "I AM", which in Hebrew is the verb "to be"... i.e., I AM the self-existing One. GOD is thus emphasizing His "aseity", thus He is the Supreme Creator and Ruler over all. So, "Jehovah" isn't His one and only name, but one of several revealed names of God.

Oh yeh... let's not forget this one too which is very applicable to dealing with JW's:

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Jeremiah 23:5-6 (KJV) "5 Behold, the days come, saith the LORD, that I will raise unto David a righteous Branch, and a King shall reign and prosper, and shall execute judgment and justice in the earth. 6 In his days Judah shall be saved, and Israel shall dwell safely: and this is his name whereby he shall be called, THE LORD OUR RIGHTEOUSNESS. (Heb: Jehovah-Tsidkenu)"
Notice that Jehovah (LORD) calls the coming Branch, a specific reference to the coming Messiah, aka: Jesus Christ, "Jehovah Our Righteousness". Jehovah calls Jesus, Jehovah. BigThumbUp

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simul iustus et peccator

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