Question on pruning maple seedling

I have a 18" maple seedling that I transplanted last summer from my old home to my new home. It's from a fantastic maple that covered the entire back yard, & the seedling started naturally under my picnic table. The original tree is about 40' tall. Zone 4.
However, it's a Y shaped seedling. My questions are:
1) Do I let it continue to grow as a Y, or do I remove one of the branches? 2) If I remove one of the branches, what season, & how old should the tree be? Will the remaining branch grow vertically, or continue growing at an angle?
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If it was mine, I'd take out one fork and stake the other to train it as straight as possible. My next door neighbor had 1/2 of a 40 year old maple come down this past summer with the Y about 4 feet above the ground. That is now one ragged looking tree.
Tom J
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