While this may be considered by some to be a Bible difficulty, I fail to see how it could be considered a contradiction. There are many thoughts as to how this should be understood. I am in line with the explanation provided at
Isaiah 7-14 Answers to Objections especially considering how the "you" in verse 14 is plural:
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"It is important to understand who this prophecy goes out to. Because Ahaz has a chance to ask for a sign (include God in the decision making process) and Ahaz refuses (Ahaz is looking somewhere else for help besides God) Isaiah's prophecy is meant for the entire "house of David"!
God is allowing King Ahaz who is king of Judah to know that Judah will survive (no thanks to Ahaz who is viewed as an unrighteous unbelieving king). While in the near term the two kings, Rezin, king of Aram (i.e. Syria) and Pekah, King of Samaria (i.e. the northern ten tribes of Israel) will not be allowed to overthrow Ahaz kingdom, a little farther down the timeline of the prophecy (65 years) Ephraim's destruction is predicted.
In the long term Isaiah prophesied to "house of David" (Judah) that Israel and Syria would be deprived of their kings before Yeshua (Jesus) would know to refuse the evil, and choose the good. Not only was Israel and Syria forsaken of both her kings, but Israel and Judah (national Israel) were forsaken of both their kings before Yeshua/Jesus knew to refuse the evil, and choose the good. At the birth of the Messiah, national Israel was under the complete domination of the Roman Empire.
Another interesting comment is that the prophecy is viewed by the Septuagint translators to render the verb "shall give" in the future tense. They felt this rendering appropriate because the sign is promised for the future, grammatically and contextually. Claims that the Septuagint tampered with the text by changing the tense to the future are sometimes made. Since the Septuagint translation was done around 285 - 244 BC it could not contain a Christian slant because Jesus had not even been born yet. The translation was done only by Jewish scribes who still felt at that time that the prophecy had not yet been fulfilled completely.(2)"
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