Should the Same Consultant Do Design and Construction Supervision?

Dear Forum member,
Don't worry. Reg. 13 (3) (b) of Public procurement ( Selection and Employment) of GN. No.98 of 2005 stipulates that, a firm or an individual who has been hired to provide consulting services for the preparation/implementation of a project, and any of its affiliates, shall be disqualified from subsequently providing goods, Works or [b]services related to the initial assignment other than continuation of the firm's earlier consulting services for the same project"
I hope your question has got cure, Coming back to merits and demerits of involving a single consultant for providing both design and supervision is as follows
Merits are:1) A consultant that was involved in design will have great understanding of the work to be executed/provided by a contractor.
Demerits are:
1) If the consultant is a man of trouble, you will continue being troubled as ready you have bidding contract with him
2) Since a single contract with be covering both design and supervision, normally consultants allocate high cost in design and then low cost in supervising. In case, you decide to terminate the contract with him. It not likely to get another supervision consultant to do that service for the remaining( retained) fund.