Q: Is it true that when a paper has been submitted to one publication, it should not be submitted to another one?

Yes, that’s true. If so, that would be a case of duplicate submission. When you submit the paper to multiple journals at the same time, that is a case of simultaneous submission. That is unethical too. You may learn more about duplicate and simultaneous submissions here: Duplicate publications and simultaneous submissions

So, you should wait to hear from the first journal before submitting to another journal. At times, you may choose to withdraw the paper from the first journal and then submit to another. But even then, you must wait for the withdrawal procedure to be completed at the first journal before submitting to another. In short, you should only submit your paper to one journal at a time.

For more help with the ethics of publishing, you may find the following courses useful:

• An overview of ethical issues in scientific research
• How to avoid retractions and publish ethically

In case you are presently waiting to hear from one journal, all the best for the next update(s) from them!