From what I have seen in literature, duodenum does express ADA. Generally ADA expresses in cell membrane. Regarding which tissues have ADA expression, here are a few articles citing expression in various tissues:
B-cell lymphoma, Pubmed ID: 6688808, 8452534
Brain, and Placenta, Pubmed ID: 15489334
Leukemic T-cell, Pubmed ID: 19690332
Thymus, Pubmed ID: 11780052