Option to prevent assigning the same user to multiple workflow steps

Feature Request: Prevent assigning a user to multiple workflow steps

Summary:
Add an option in the proposal workflow settings to prevent the same user from being assigned to multiple workflow steps on the same PBS/Task. This ensures clear separation of duties between roles such as writer and reviewer.

Problem Statement:
Currently, a single user can be assigned to multiple workflow steps (e.g., both writer and reviewer) on the same PBS/Task. This creates a conflict of interest, reduces the effectiveness of reviews, and undermines process controls that rely on independent verification. It can also introduce compliance and audit concerns where separation of duties is required.

Proposed Solution:
Introduce a configuration option on the workflow tab of the proposal settings that, when enabled, enforces unique user assignments per PBS/Task across all workflow steps.
If a user is already assigned to one step for a given PBS/Task, the system should prevent assigning that same user to another step on that PBS/Task and display a clear validation message.
Provide an override or admin-only control (if applicable) to temporarily bypass this restriction, with appropriate warnings.
Support this behavior consistently in both the UI and any relevant APIs or bulk assignment tools.

Benefits:
Ensures separation of duties between roles (e.g., writer vs. reviewer) for the same PBS/Task.
Improves review quality by requiring independent reviewers.
Reduces the risk of process, compliance, or audit issues where independent validation is required.
Provides clearer guardrails for users, reducing assignment mistakes and rework.