What are the dangers of doubling up on vit d3?

Likely no problems: Doubling the recommended dose of vitamin D3 is not likely to cause any issues, especially if one is low on this vitamin. See your doctor to have your vitamin D3 level checked.

Possible toxicity: It's unclear what is meant by "doubling up". The "recommended dietary allowance" (rda) ranges from 400 iu to 800 iu per day, but some people may need up to 2000 iu per day of a supplement other than dietary sources. Serum levels of 30-50 nmol/l are recommended and toxicity can occur, so the daily dose should be adjusted according to lab results. Common toxic effects are anorexia & kidney stones.

Watch the dose : Too much if anything is not good. If you take too much vitamin d, you may develop: vomiting, polyuria (frequent urination), weight loss, poor appetite, constipation, weakness, heart rhythm abnormalities, kidney stones. Evidence suggests >4000 iu daily can be detrimental. Most ppl don't need more than 800 iu. Ppl that are in the sun >15 min daily might not need any supplements at all.