The meaning of doubled and multiplied
The detailed rules regarding 'riba' and the philosophy underlying its prohibition have been discussed thoroughly in Al-Baqarah 2: 275⧉ 276⧉ 277⧉ 278⧉(Volume 1 of this commentary). However, it may be pointed out here that words أَضْعَافًا مُّضَاعَفَةً of (doubled and multiplied) used in this verse Ali 'Imran 3:130 ⧉ do not mean that the prohibition of 'riba' is restricted only to a transaction where the interest is doubled or multiplied. In fact, these words are not used in a restrictive sense, because it is evident from Surah al-Baqarah⧉ that 'riba' or interest is prohibited in any case, even though its rate is not so high as to make it doubled or multiplied.
These words are used only to explain the factual position prevalent at that time, and to indicate its unjust and evil nature. Moreover, these words also suggest that even the interest charged is simple and not compound, yet once a person is engaged in the business of interest he does not stop at charging interest in one transaction only. Rather, he reinvests the income of interest in another loan transaction, and keeps investing the interest proceeds in similar transactions again and again, and thus the ultimate result is that the original interest charged through the first transaction is doubled and multiplied.