Vitamins and minerals from foods an supplements taken simultaneously can be absorbed normally, because their absorption is usually not regulated by the amounts you take. However, the absorption of iron depends on your body iron stores: the more stores, the less efficient absorption. Still, both acute and chronic iron poisoning is possible with large amounts of iron supplements.
The absorption of selenium is not regulated like iron, so overdose is possible, but not with balanced diet and supplements within recommended limits. Note, that selenium toxicity can occur if you eat too many Brazil nuts (ODS).
To know, if it is better to take a supplement on an empty stomach or with food, read the instructions on the information leaflet provided. Fat-soluble vitamins (A, D, E, K) are better absorbed if taken with fatty food (PubMed).
NHS Inform about taking higher than normal amount of supplements:
If you get too much water soluble vitamins (B complex and C) and minerals, the excessive amount will be usually excreted by the urine within hours. Overdose can occur when you exceed the Tolerable Upper Intake Level for these vitamins.
If you get too much fat-soluble vitamins (A, D, E, K), they will be stored in the liver and other body fat stores and used later at any time if needed. Taking these supplements within recommended doses even in addition to balanced diet and fortified foods, does not result in an overdose (PubMed).
Good to know: Vitamin and Mineral Supplements: Do We Really Need Them? (PubMed Central, 2012)
The results of large-scale randomized trials show that, for the
majority of the population, there is no overall benefit from taking
Multivitamin supplements.
