filter unique alphabets

Shell Programming and Scripting
1

Filter unique alphabets (bold) from input

Thanx

+SRR015270.1 HWI-B10_3_6069:2:1:653:875 length=32
SZZZZZZZZZZZZXZZZXZZZOECZZIZHUEM
+SRR015270.2 HWI-B10_3_6069:2:1:455:450 length=32
ZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZTT
@SRR015270.3 HWI-B10_3_6069:2:1:453:499 length=32
ZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZTT
@SRR015270.4 HWI-B10_3_6069:2:1:453:499 length=32
ZZZ
@SRR015270.5 HWI-B10_3_6069:2:1:453:499 length=32
ZZZ

output

+SRR015270.1 HWI-B10_3_6069:2:1:653:875 length=32
SZZZZZZZZZZZZXZZZXZZZOECZZIZHUEM
+SRR015270.2 HWI-B10_3_6069:2:1:455:450 length=32
ZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZTT
@SRR015270.4 HWI-B10_3_6069:2:1:453:499 length=32
ZZZ
2 
awk '{x=$0;getline;if (a[$0]==0){print x"\n"$0;a[$0]++}}' file
3

Another one:

awk 'NR%2{p=$0;next}!($0 in a){print p RS $0;a[$0]}' file