Publishing in multiple journals is heavily frowned on (and most journals will not republish work irrespective of copyright) since it is normally done to (a) boost publishing records and give the impression of greater productivity (CV padding) & (b) increase the authors' citations (i.e. the impact of the work) & (c) add undue weight to a hypothesis, by increasing the apparent number of papers that support it.
However, this is not a barrier to including papers in institutional repositories etc, simply to publishing in multiple journals.
However, this is not a barrier to including papers in institutional repositories etc, simply to publishing in multiple journals.