The issue of duplicate publication in academic research refers to the act of re-republishing research-related material already available in print or online.
The seriousness of this offence as a breach of code increases from text-recycling as part of plagiarism to redundant publication of work published previously elsewhere under copyright.
Conference Abstract Reuse
Commonly used plagiarism software by academic journals will highlight overlap with an abstract of the same study, previously published in conference proceedings and therefore available online.
Most journals will interpret this overlap as acceptable if the size of the overlap does not exceed the length of an average conference abstract (generally 400 words maximum). Publishers may have slightly different permission policies on this topic and you are recommended to check the details of what degree of reuse allowed here.
If in doubt, electronic requests for (free) permission can nowadays be initiated through most journal's websites.
Salami Slice Publication
In academic publishing, salami slicing is a term used for the publication of a journal article directly and closely related to another journal article, which questions the appropriateness of two separate publications on the topic. This repetition falls in the grey area of questionable research practices.
Separate publication of two directly and closely related publications is generally considered acceptable if the overlap concerns the same methodological approach or use of a data set. Under these circumstances, overlap is understood to reflect specialist expertise and/or appropriate use of an available data collection.
Particularly the use of the same data collection as a resource for different research projects is considered acceptable when different research questions are addressed in the overlapping research articles. An appropriate example would be the justified use of a population cohort database for more than one publication. This approach would meet the moral obligation to prevent underutilisation of data collected from study participants, who voluntarily contributed their time, personal information and/or bodily samples for research.
In contrast, attempts to split research findings from the same series of experiments or single data analysis into more than one publication may cross the line. It likely depends on the journal whether in some cases a new submission for publication is perceived by the editor as salami slicing.
Awareness of the phenomenon being regarded as inappropriate at times can help a researcher being transparent and bringing sufficient nuance in the new publication to emphasise its added value and significance. This can be accomplished in the manuscript itself or addressed in the cover letter to a journal editor at manuscript submission for publication.