Hi Ed,
When importing requirements, there is actually a scenario where the tool does not check to see if there are duplicate names, and potentially will import some of your elements as duplicates. Including an ID column in your import file is optional, but if you have an ID column defined, this supersedes any names you have entered and the software will compare only the ID values in the file to what exists in your project. Therefore, if you have blank IDs combined with a name that already exists in your project, it will treat the imported element (that has no ID) as a separate item and create a "duplicate" with the same name in your project. To avoid this, you can remove the ID column from your import file, and the software will compare the imported items solely based on their names.
The second scenario you describe is also understandable. When importing from an Excel file, the software can't tell what sort of elements you're bringing into your project. Importing relies entirely on your input to decide what type of elements you're bringing in. If you do make a mistake, the undo button (or Ctrl + Z) will reverse the import, and you can perform it again and select the correct element type.
In regards to disabling different requirement types, yes and no. To explain, you can most definitely restrict what types of requirements can be entered in your project through the Built-in Fields option. Click Tools / Built-in Fields on the ribbon bar, and then you can choose which requirement types can be added by changing the value options, including the "Allow only these values" checkbox (see attached). However, this will not limit the types of requirements that can be added when importing Excel files, as requirement types that are restricted in your project can still be imported manually.
If you have any further questions or would like any additional help, please let us know.