How can different end nodes output the same key variable?

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### Dify version

1.15.0

### Cloud or Self Hosted

Self Hosted (Docker)

### Steps to reproduce

Set two end nodes with the same input key

### ✔️ Expected Behavior

the workflow able to publish

### ❌ Actual Behavior

Error: A variable cannot be duplicated, causing the workflow to be unable to publish

Start by reproducing the issue in the self-hosted Docker deployment with two end nodes using the same input key. Trace the workflow publish validation to determine why duplicate keys are rejected; the work is done when this workflow publishes successfully without breaking duplicate-variable validation elsewhere.

Written by the indexing model from the issue text.