In answering this question, I wondered:
Many fields are now beginning to embrace preprint servers such as arXiv. Typically, authors upload a manuscript to the preprint server at the same time as they submit it to a journal. On the whole, this is seen as acceptable, and not as 'dual publication'.
On the other hand, submission of the same manuscript to two 'proper' journals simultaneously is (in all fields I know) regarded as serious misconduct.
Where, precisely, is the boundary between 'acceptable' and 'unacceptable' conduct here? What distinguishes a 'preprint server' from a 'journal'? At first glance, one might say 'peer review'. However, journals come in many forms, including 'open review' (where the submitted manuscript is available for anyone to read and comment on immediately), and 'predatory' (where any submission can be accepted without review if you pay the fee). Similarly, many recognised preprint servers exercise some basic quality control on submissions.