Objection 1. It would seem that a solemn penance can be repeated. For those sacraments which do not imprint a character, can be solemnized a second time, such as the Eucharist, Extreme Unction and the like. But Penance does not imprint a character, therefore it can be solemnized over again.
Objection 2. Further, penance is solemnized on account of the gravity and publicity of the sin. Now, after doing penance, a person may commit the same sins over again, or even more grievous sins. Therefore the solemn penance should be imposed again.
On the contrary, Solemn penance signifies the expulsion of the first man from paradise. Now this was done but once. Therefore solemn penance should be imposed once only.
I answer that, Solemn penance ought not to be repeated, for three reasons. First, lest frequency bring it into contempt. Secondly, on account of its signification; for it signifies the expulsion of the first man from paradise, which happened only once; thirdly, because the solemnization indicates, in a way, that one makes profession of continual repentance. Wherefore repetition is inconsistent with solemnization. And if the sinner fall again, he is not precluded from doing penance, but a solemn penance should not be imposed on him again.
Reply to Objection 1. In those sacraments which are solemnized again and again, repetition is not inconsistent with solemnity, as it is in the present case. Hence the comparison fails.
Reply to Objection 2. Although, if we consider his crime, he ought to do the same penance again, yet the repeated solemnization is not becoming, for the reasons stated above.
Source: Summa Theologica (New Advent)