You can clip off all but the strongest shoot. Are they exposed to light? That would help develop the shoots and the main shoot would announce itself. The pro's say new shoots will emerge from the tuber. I've never tested it.
Here is a portion of a paragraph from CTdahlia.org and I hope it explains it better than I tried to do!
"Growing: Dahlias should be grown as a single stalk, with multiple shoots removed as they emerge from the ground. The growing tip is removed (pinched out) when the plant reaches 6-10 inches in height or has 5-7 sets of leaves. This will promote lateral growth (shoots) and multiple blooms. Each lateral will produce a main (terminal) bud and two adjacent buds."
