how to ensure only unique values in dropdown

I've got a drop down with a bunch of values and some of them are dupes:

                        <label for="AAAPurposeType_AAAPurposeType" class="bl-label bl-text-align--left">
                            <%=Html.Encode(Model.AAALoanType.DisplayName)%></label>
                        <div class="data width-70"><%=Html.EditorFor(m => m.AAALoanType)%></div>

How do we display ONLY unique values?