William Durandus explains why the Introit of the Eighth Sunday after Pentecost is repeated from the feast of the Purification.
“There follows the Eighth Sunday, on which the Church teaches us to avoid all vanity. For this must be the effect of its teaching within us, because in its teaching it teaches us to become spiritual men, and be removed from bodily desires, unto the likeness of the Blessed Virgin, whose feast (i.e. the Assumption) is approaching. For this reason the Introit begins, ‘We have received Thy mercy’, that is, Thy Son, Jesus Christ, given to us out of mercy, ‘in the midst of Thy temple’, that is, in Thy universal Church. ‘According to Thy name, o God’, for God is named everywhere; ‘so also is Thy praise unto the ends of the earth’, that is, everywhere. For the ‘temple’ is also the Blessed Virgin, in whom we have truly received the mercy of God; wherefore, reasonable do we sing the current Introit around the time of Her feast, since she is the temple of the Lord, and the sanctuary of the Holy Spirit.” (Rationale Divinorum Officium, VI.122.1)