How to prevent approval of own expenses

Summary:

We have the following scenario:

1. Marianne is creating an expense on behalf of Jane and submits it
2. Jane is receiving an approval notification and approves it
3. Expense is routed to Pernille. Pernille have got a vacation rule (all approvals are sent to Jane) and expense is rerouted to Jane
4. Jane approves the expense - again!

Jane has now approved her own expense. How can this be avoided?

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