A single sentence can contain one or two 了 le.
Verb + 了 le (= suffix)
The action is completed. I have stopped.
| 我 | 学 | 了 | 三 | 年 | 。 |
| wǒ | xué | le | sān | nián | _ |
| I | study | particle | three | year | _ |
| I, me, my | study, learn | grammatical particle of time | three | year, surname | 。 |
| I have studied three years. | |||||
Verb + 了 le (= suffix) + time + 了 le (= particle)
The action is completed but only concerning the three years. I am still studying.
| 我 | 学 | 了 | 三 | 年 | 了 | 。 |
| wǒ | xué | le | sān | nián | le | _ |
| I | study | particle | three | year | particle | _ |
| I, me, my | study, learn | grammatical particle of time | three | year, surname | grammatical particle of time | 。 |
| (Up to now,) I have studied three years. | ||||||