Throw an error if the user tries to register two models with the same name

Issue Creation Checklist

• I understand that my issue will be automatically closed if I don't fill in the requested information
• I have read the contribution guidelines

Feature Description

Describe the feature you'd like to see implemented

Make the following impossible:

sequelize.define('User');
sequelize.define('User');

The second define should throw an error, because a model named User already exists in sequelize.models.

Describe why you would like this feature to be added to Sequelize

This behavior is dangerous and a source of bugs. Users should remove models manually, and we should ensure that that is possible (i.e. nothing depends on that model), before being able to register a model with the same name.

Example of an issue caused by this: https://github.com/sequelize/sequelize/issues/15639

Is this feature dialect-specific?

• No. This feature is relevant to Sequelize as a whole.
• Yes. This feature only applies to the following dialect(s):

Would you be willing to resolve this issue by submitting a Pull Request?

• Yes, I have the time and I know how to start.
• Yes, I have the time but I will need guidance.
• No, I don't have the time, but my company or I are supporting Sequelize through donations on OpenCollective.
• No, I don't have the time, and I understand that I will need to wait until someone from the community or maintainers is interested in implementing my feature.

Indicate your interest in the addition of this feature by adding the 👍 reaction. Comments such as "+1" will be removed.

Start at sequelize.define and inspect how models are registered in sequelize.models. Use the duplicate User definition and issue #15639 as context. Done means the second registration throws an error while allowing models to be removed and registered again when appropriate.