interaction design – Combining single- and multi- select drop-down in one form. How to dismiss the drop-down?

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How to dismiss the drop-down in single-select and multi-select cases?

For single-select, the obvious option is to dismiss the drop-down on selection.

For multi-select, drop-down should stay open on selection, and can be dismissed with a button, or clicking outside.

Question: Will it be confusing for people to have two models of interaction side-by-side? What is the best strategy here?

Mind that cost of error ((wrong selection) is small. Nothing is saved, or processed, or launched into the space when option is selected.

Examples from the web are welcome in the comments section.