Exclude duplicate values in certain columns using ddply
I have a data frame with the following structure: > dftest element seqnames start end width strand tx_id tx_name 1 1 chr19 58858172 58864865 6694 - 36769 NM_130786 2 10 chr8 18248755 18258723 9969 + 16614 NM_000015 3 100 chr20 43248163 43280376 32214 - 37719 NM_000022 4 1000 chr18 25530930 25757445 226516 - 33839 NM_001792 5 10000 chr1 243651535 244006584 355050 - 4182 NM_181690 6 10000 chr1 243663021 244006584 343564 - 4183 NM_005465 1316 100302285 chr12 12264886 12264967 82 + 24050 NR_036052 1317 100302285 chr12 9392066 9392147 82 - 25034 NR_036052 1318 100302285 chr2 232578024 232578105 82