Hi.
A thickened dura can be caused by many conditions including inflammatory/infective conditions (tuberculous meningitis, sarcoidosis, lyme disease, syphilis), neoplastic disorders (carcinomatous meningitis, meningioma en plaque), collagen vascular disorders (rheumatoid arthritis, Wegener's granulomatosis, mixed connective tissue disease), and other miscellaneous conditions (spontaneous intracranial hypotension, mucopolysaccharidosis, chronic intrathecal drug administration, intracranial bleeding).
The significance of this finding would depend on your history and current symptoms.
If you are concerned about this, you should consult a doctor about it.
Hope this answers your query.