Added a warning message in `Upload.cshtml` to notify users when a duplicate invoice is detected. Introduced the `IsDuplicate` property in `UploadModel` to track duplicates and updated the `OnPostAsync` method to set this property based on the `ImportedAt` timestamp.
Enhanced the `ImportAsync` method in `ZugferdImportService` to include duplicate detection by checking the database for invoices with the same `InvoiceNumber` and `SellerTaxId`. If a duplicate is found, it logs a warning and returns the existing invoice.
Updated `ImportAsync` to accept an optional `guidelineId` parameter and added logging for duplicate detection and successful imports.
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