"in crowning their merits, you crown your own gifts."

It is truly right and just, our duty and our salvation,
always and everywhere to give you thanks,
Lord, holy Father, almighty and eternal God.

For you are praised in the company of your Saints
and, in crowning their merits, you crown your own gifts
     [(et eorum coronando merita tua dona coronas)]....

And so, with the Angels and Archangels, and with the great multitude of the Saints,
we sing the hymn of your praise,
as without end we acclaim:

Holy, Holy, Holy. . . .

     Preface I of Saints, all italics mine.  This Preface and the sentence that I have italicized derive from prayer no. 1258 in (on p. 289 of) the Missale Parisiene of 1789 (also a Preface of Saints), where the sentence in question appears in a slightly different word-order:  "et eorum coronando merita, coronas dona tua."  I don't know how far back this prayer goes in the Parisian tradition, but according to pp. 445-450 of The Prefaces of the Roman Missal, by Ward and Johnson, where it is Preface no. 69, the sentence in question derives from De gratia and libero arbitrio 15 (PL 44, 890-891) and Tract. in Ioannem 3.10 (PL 35, 1401) of St. Augustine, where the wording is very similar.  The Catechism of the Catholic Church cites (at the head of the section on Merit, i.e. just before #2006, on p. 541n59) also En. in Ps. 102.7 (PL 37, 1321-1322).
     Here the new translation certainly did improve things.  For the old one read more ambiguously, "You are glorified in your saints, for their glory is the crowning of your gifts."