[asset_path_payment] No check that path entries are unique addresses

[Wilfred007]

Description

The path-payment routing logic in complete_path_payment builds a Vec<Address> representing the conversion path but never checks for duplicate addresses within that path. A caller could submit the same intermediate asset twice, which doesn't fail outright but produces a confusing or misleading path that doesn't reflect a real conversion route.

Where

contracts/asset_path_payment/src/lib.rs (path validation block, ~line 148-158)

Acceptance Criteria

• Reject payment initiation when the path vector contains a duplicate address
• Add a unit test covering the duplicate-path rejection case
• Document the constraint in the function's doc comment

[leocagli]

Hi, I'd like to add a uniqueness check for path entries in asset_path_payment. Guards against duplicate addresses in the swap path. Could you assign me?