Words with double I's were accepted during the Augustan era. During the Ciceronian era, however, this construction was either rare or unheard of. <br /><br />Augustan- Consilium, Consilii<br />Ciceronian- Consilium, Consili<br /><br />Augustan- Harrius, Harrii<br />Ciceronian- Harrius, Harri<br /><br />Since they are both regarded as Classical eras, you can choose whichever pattern you like. Most books, though, teach the Augustan rules, and you will be more likely to be understood by Latin speakers.
flebile nescio quid queritur lyra, flebile lingua murmurat exanimis, respondent flebile ripae