What is the difference between hsa-miR-33a and hsa-miR-33b?
They are paralogs i.e. they have high level of sequence homology. They may have the same seed sequence too (see this). The paralogs can have same targets if they have same seed sequence but it is not essential in all cases. However, the extent of homology required for this kind of classification is poorly defined. miRBase still follows the guidelines mentioned in this (quite old) article:


  When such a candidate is so similar in sequence to the known miRNA
  that the probe designed to detect it would surely cross-hybridize with
  the known miRNA, criterion A need not be satisfied, and the candidate
  can be annotated as a variant form of the known miRNA, provided that
  the candidate itself meets criterion C, and there is also very high
  confidence that one of these paralogs is a confirmed miRNA (i.e., its
  classification is supported by at least three of the listed criteria).
  When there are two or more mismatches between the miRNA paralogs such
  that probes could differentiate between the species on Northern blots,
  criterion A must be satisfied experimentally. Sometimes partially
  overlapping cDNA sequences are identified from the same locus; these
  are assumed to represent differentially processed products from the
  gene.