Thomas Aquinas On Original Sin, Part 6.

From Summa Theologica:

Treatise on Habits in Particular, Question 82, Article 2: Whether there are several original sins one man?

Aquinas considers the possibility that there are several original sins in each person due to: (1) a particular translation of Psalm 51:5 which reads "in sins did my mother conceive me," (2) people are habitually inclined [see the last article] to contrary sins, (3) every part of the soul is infected by sin, and so the different parts [it seems Aquinas has in view the "intellect," the "will," etc.] may be infected by different original sins.

Aquinas responds that there is only one original sin present in each person and that this is "the first sin of the first parent that is transmitted to posterity."

Aquinas argues that the loss of "original justice" [again, a rightly ordered inner life, submitted to God] accounts for points 2 and 3 above. As for point 1, Aquinas simply asserts that Scripture frequently substitutes plurals in the place of singulars for various reasons.

Again, Aquinas's conclusions seem plausible, but his argument lacks any scriptural exegesis (in his positive argument, he only cites John 1:29, and this is used in a somewhat dubious manner) that would prove his case beyond doubt.