/r/linguistics may have the expertise needed to answer this question.

In a sense, there always has been. Even in Modern English, you'll notice the preposition to has a stressed /tu:/ and an unstressed /tə/ pronunciation.

A similar phenomenon happened in Old English. "To" served as both a preposition and an adverb, and there was a stressed version /to:/ and unstressed version /tɔ/. As an adverb, it was almost always stressed, but as a preposition, it was much more frequently unstressed in connected speech.

By the 16th Century, the stressed version eventually came to be written as "too" to reflect the difference in pronunciation and took on an independent identity as a full-time adverb.

The vowels shifted more over time, of course, but that's essentially how we got to where we are today. The stressed "to" is still homophonous with the always stressed "too" but the unstressed "to" is not.