To avoid unexpected behaviour/errors, users should not be able to add duplicate activities to a workflow

_This issue is a follow-up to a comment posted by @sfmskywalker in https://github.com/elsa-workflows/elsa-core/issues/683#issuecomment-798898997_

Currently, it is possible to create, execute & persist a workflow which contains activities that have duplicate IDs. However, this will almost certainly be troublesome in some scenarios and cause errors when `GetActivityById` is used.

This issue is to ensure that scenarios in which users try to add duplicate (same `Id`) activities to workflows are detected and prevented, such as by an exception.
* Test using the UI designer
* Test using coded workflow definitions
* Test using JSON/imported workflow definitions
* Test using existing persisted workflows (it's possible that users might have created & saved some before this issue gets done)