Hii

Yes you can appear in April session of JEE Mains Again even if you had  apeared in january session.
The best of the two scores would be used for the final results and further processing.
The both attempts of the particular year would be counted as 1 and a total of 3 attempts for 3 consecutive years would be allowed i.e total 6 times you can write the paper.
refer to the following link for more detail
https://engineering.careers360.com/articles/jee-main-number-of-attempts

Hope this helps
Thanks!!