Why does the pET- expression vector contain a LacI gene additionally to the one in the genome?
According to this article, a single lacI gene copy gives rise to about 10 copies of lacI protein per cell, and we can conclude, therefore, that this is the amount required to keep a single lac operon repressed.  The article also mentions the lacIQ mutation in the promoter of lacI that results in a ten-fold increase in the level of lacI protein. 

If a lac expression construct is present at more than 100 copies per cell the result will be leaky repression, and this can result in selection for mutations in the gene of interest if it has deleterious effects on the host bacteria. The easiest way around this problem is to put a lacI (or lacIQ) gene on the expression plasmid so that repressor levels are commensurate with the number of copies of the lac operator, as in the pET vectors.