Residential ducts are rarely sealed very well. If you want to be charitable, I guess you could say that as long as the ducts are within the conditioned space, it's not really a big deal because the hot/cold air is still within the house. But then, even where they run ducts in unconditioned space (attics or vented crawlspaces), they still rarely do a good job sealing them.
If you do want to seal the gaps, it's super easy. Just get some foil tape and cover any gaps you have access to. Or if you have a bunch of awkward shapes that'll be hard to tape, you can also use duct mastic. It's basically a gooey peanut butter consistency material that you just spread over the gaps. It works great, but it is a bit messy and probably harder to find than foil tape, so tape is simpler.
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