The tax code does not permit you to make an HFD to fund a previous year's HSA contribution. The HFD that you made in March 2018 must be treated as a 2018 HSA contribution.
The contribution that you made in December is an excess contribution to the extent the sum of the March 2018 HFD plus the December contribution exceeds the limit for 2018.
If you somehow treated the March 2018 HFD as a 2017 HSA contribution on your 2017 tax return you'll need to amend your 2017 tax return.
