First you need to state whether you are boiling filtered water or unfiltered tap water.

1. Filtered tap water
Most filtration system are reverse osmosis. This removes almost completely the minerals. Boiling almost pure water over and over again gives you pure water.

2. Unfiltered tap water
Reboiling increases the concentration of the minerals due to loss of water molecules as steam. Is the amount of mineral consumed harmful? That depends on your overall diet. It might even be a supplement for your deficiency in rare cases.