Is it necessary to re-confess sins from your last confession if it was invalid?

[christianguy]

I am aware that to deliberately withhold a mortal sin in confession is a mortal sin itself and it’s a sin of sacrilege and I know that we must confess all mortal sins for a confession to be valid. But suppose (this is hypothetical) that I have committed 10 mortal sins since my last confession and when I go to confession I only confess 9 of those 10 mortal sins.

I know my confession would be invalid but how would I approach my next confession? Apart from saying that I withheld a mortal sin and describing that mortal sin, should I also re-confess the other 9 mortal sins? I think this is correct however one of my priests say that in that case one should mention the unconfessed mortal sin but it is not necessary to re-confess the other mortal sins. Who is right? I would also appreciate a quote from the Catechism or from a Church document from the Magisterium which clarifies this.

 

[Fr.Charles_Grondin]

Baltimore Catechism:

Q. 798. What must he do who has willfully concealed a mortal sin in Confession?

A. He who has willfully concealed a mortal sin in Confession must not only confess it, but must also repeat all the sins he has committed since his last worthy Confession.