Issue with manifest items being declared multiple times.

It seems that manifest items are being duplicated when adding files using the newly added Drop Zone.

Steps to Reproduce:

1. Add a file via the Drop Zone.
2/ Add another file with the exact same name via the Drop Zone again.

When adding files this way, two manifest items are created: one using the filename ID and another using a random ID.

Example of duplicate manifest items generated by Sigil.

<item id="txt_69-2.png" href="Images/txt_69-2.png" media-type="image/png"/>
<item id="x9dbbd7d2-c149-4d38-9e87-21079c863550" href="Images/txt_69-2.png" media-type="image/png"/>

Note: For reference, I am using a custom build from the latest source code, which includes the fixes for the prettify issue and the CSS synchronization issue.