Augustine's "double procession" theory was the origin of the filioque, in his De Trinitate.
Most everyone agrees with this, Hermann, except you. Try a google search for "double procession".
St. Augustine systematized what had long been taught. For example:
"Concerning the Holy Spirit I ought not to be silent, and yet I have no need to speak; still, for the sake of those who are in ignorance, I cannot refrain. There is no need to speak, because we are bound to confess Him, proceeding, as He does, from Father and Son" (St. Hilary of Poitiers, On the Trinity, 2.29 AD 357)
You might note that date predates St. Augustine's baptism. We've been down this road before.