Presenting a paper at a conference/seminar that is currently under review by a journal
There are two aspects to your question:


“Duplicating” a publication by having both a conference paper and a journal paper about it.

This has been discussed here before, and a short summary is that this usually not a problem: apart from Computer Science, where conference papers have a rather different status that in other fields, “publishing a paper does not normally prevent you from presenting your work at a conference”.
Presenting as-yet-unpublished results in a conference/seminar.

There's nothing wrong about that at all, and I think more people will actually enjoy you discussing recent results rather than old stuff. Customs, here again, depend on your field: I've seen more people discuss unpublished results in physics, and fewer in chemistry… but even in fields where it is less common, it is not forbidden.

One thing to note: if the paper is not yet published, it is probably polite to inform your co-authors (or ask for their permission, depending on your relationship) that you are going to present it at a conference. At least, that's what I do (and expect my co-authors to do).