An individual spell or ability can only target a single object once for each use of the word target in a card’s text. A card like like Seeds of Strength is a good example of a card where you can target the same permanent multiple times. It can give three different creatures +1/+1, one creature +1/+1 and another+2/+2 or a single creature +3/+3.

However there are some spells or abilities, like Leeching Bite, that use the word ‘target’ more than once, but also specify that each target must be different. With these spells and abilities you cannot target the same player or object more than once even though they do use the word target more than once.