Why is lead(II) present in two groups of cation analysis?
Simply because lead(II) fits both analytical groups:


it forms lead(II) chloride $\ce{PbCl2}$, a poor soluble chloride, which puts it in the 1st group;   
it also forms lead(II) sulfide $\ce{PbS}$, insoluble in water and diluted acids, which also puts it in the 2nd group.


One reason for putting lead in both groups is that the chloride is somewhat of a borderline case in terms of being insoluble enough to appear in Group I.  Group II represents a more complicated analysis but is more sensitive for lead.