Find duplicated text for tasks in MS Project
If I understand correctly, you are testing to see if there are any duplicate entries in Text30 and if you find a duplicate setting flag 5 to 1.

In this case you are duplicating at least half of the tests that you are conducting because Task(1).Text(30) = Task(2).Text30 is the same comparison as Tasks(2).Text30 = Task(1).Text30.

So you can cut down on the number of tests that you do by eliminating tests that you have already performed.  This is done by starting the inner loop at the current value of the outer loop.

e.g. 

For Temp2 = 1 To ActiveProject.Tasks.Count


should be revised to

For Temp2 = Temp To ActiveProject.Tasks.Count


However an even quicker way would be to use a Scripting.Dictionary. I don't have project installed on my PC but even so if the code below doesn't work it should at least point you in the right direction

Sub Lacunza()

    Dim myUniqueText30 As Scripting.Dictionary
    Set myUniqueText30 = New Scripting.Dictionary

    Dim myTask As Long
    For myTask = 1 To ActiveProject.Tasks.Count

        With ActiveDocument.Tasks(myTask)

            If myUniqueText30.Exists(.Text30) Then

                .Flag5 = 1

            Else

                myUniqueText30.Add Key:=.Text30, Item:=myTask

            End If

    Next

    ' As this point you have a scripting dictionary which will return the task id
    ' where the first occurence of a particular Text30 value was found.

End Sub