Is it OK to make multiple observations of the same species?

Yes! It's fine to make more than one observation of the same species and even do so on the same outing, or in the same area over time. Don't feel obligated to do so, but it's OK on iNaturalist.

However, do keep in mind that each observation is effectively a request to the community to evaluate it and add identifications or comments to it. A basic and useful approach is to make a few representative observations of the population, especially observations that capture things like phenology (such as an individual plant that's flowering, another individual that's fruiting), or different behaviors (like bees in different flowers).