Implement Duplicate Transaction Protection

**Description:** 
Prevent duplicate donation transactions from being processed.

**Tasks:** 
- \[ \] Store transaction hash 
- \[ \] Check existing hash before processing 
- \[ \] Reject duplicate transactions - 
- \[ \] Emit error event 
- \[ \] Test duplicate scenarios

**Acceptance Criteria:**
 - Duplicate transactions rejecte- 
 - No double-counting occurs - 
 - System remains consistent