This is fairly common, in my experience, at top machine learning (and adjacent) conferences. Usually, the non-archival venue is a workshop attached to some conference, so the repeat submission typically happens in one of the following ways:
- Early version of paper is submitted to a non-archival workshop A, and then to an archival venue B (e.g. a conference). This could also happen if the paper was first rejected at the main track of the conference which A is attached to.
- Paper is accepted at main track of a conference (archival), and then submitted to a workshop (non-archival) attached to the same conference, whose focus area is the subfield the paper belongs to. Some workshops may accept such papers without additional review.
- A previously published paper at an archival venue is later submitted to a workshop that focuses on its subfield.
The idea behind additionally submitting to a non-archival workshop is typically to allow more detailed presentations (e.g. an oral presentation as compared to "just" a poster at a larger conference), more detailed discussions, more visibility for the paper, and to an audience that primarily is from the same subfield (as compared to a broader audience at large conferences).
I am not sure if submitting to an archival and non-archival venue simultaneously is common, but I do not see any ethical issues, beyond two primary considerations:
- As other answers say, you should check the policies of the archival (and non-archival) venue. For example, NeurIPS allows submitting papers that have previously appeared in non-archival venues:
Work that has appeared in non-archival workshops, such as workshops at NeurIPS/ICML, may be submitted.
CVPR, on the other hand, does not:
A publication, for the purposes of this policy, is defined to be a written work longer than four pages (excluding references) that was submitted for review by peers for either acceptance or rejection, and, after review, was accepted. In particular, this definition of publication does not depend upon whether such an accepted written work appears in a formal proceedings or whether the organizers declare that such work “counts as a publication”.
Additionally, some venues may allow such submissions in general, but may restrict them during their review period.
- If the submission to the non-archival venue is after the archival paper has been published (with copyright transfer), you might need to check the policies of the publisher regarding this.
If it gets accepted by both, you will get the chance to present at both. Formal proceedings and copyright transfer would only take place for the archival venue, and that version would typically be considered "canonical" when listing on publication lists and for citing the paper. In particular, assuming the two are nearly identical, you do not get to count them separately in your list of publications.
The question of submitting to arXiv is a orthogonal to this discussion, as arXiv is an archival "venue", more than most non-archival workshops, but by not being peer-reviewed, does not have the ethical concern of wasting reviewer time or questions regarding copyright transfer.