Multi-submission of one manuscript refers to the act of submitting the same paper to two or more journals at the same time without notifying the editors of the journals.

There are two reasons why multiple submissions are considered to violate publishing ethics:

If more journals accept the publication of the same paper, it will cause copyright disputes.

When resources are wasted, there will be more periodicals, and it will take time and money to do the same work on the same piece of paper.

Multiple submissions may result in:

Regular rejection

Blacklist the author and prohibit future contributions.

The periodical informs the author's unit.

References:

Repeated publication, multiple contributions for one manuscript? /insights/duplicate-publication-and-simultaneous-submission