merge multiple mp4(h264) video the time frame get reduce by 10%

Hi all
Pardon for my English language, when I try to merge mp4(h264) video the time frame get reduce by 10% for example. If I have two video of 15 second each. When I combine it has to give me 30 sec. but the final output video length is 27 sec only. But by comparing the original video and merge video both are equal.

While merging the video I got list of error like skipping frame

Pos: 52.4s 1452f (86%) 17.63fps Trem: 0min 39mb A-V:0.084 [5459:3]
Skipping frame!
Pos: 52.7s 1462f (87%) 17.74fps Trem: 0min 39mb A-V:0.084 [5459:3]
Skipping frame!
Pos: 53.1s 1472f (88%) 17.84fps Trem: 0min 39mb A-V:0.084 [5463:3]
Skipping frame!
Pos: 53.4s 1482f (97%) 17.95fps Trem: 0min 35mb A-V:0.084 [5454:3]
Skipping frame!
Pos: 53.8s 1492f (98%) 18.05fps Trem: 0min 35mb A-V:0.084 [5456:3]
Skipping frame!
Pos: 54.2s 1502f (99%) 18.15fps Trem: 0min 35mb A-V:0.084 [5464:3]
.......


I am clipping the video using ffmpg and merging the video using mencoder.

ffmpeg query for clipping the wmv video file and convert it to mp4
================================================== =================

ffmpeg.exe -ss 00:34:2.30 -t 00:00:15.00 -i "F:\GAMES\Sample.wmv" -r 25 -b 3000k -croptop 0 -cropbottom 0 -cropleft 0 -cropright 0 -y "C:\output1.mp4"
ffmpeg.exe -ss 00:54:2.30 -t 00:00:15.00 -i "F:\GAMES\Sample.wmv" -r 25 -b 3000k -croptop 0 -cropbottom 0 -cropleft 0 -cropright 0 -y "C:\output2.mp4"

Merging query in mencoder
======================
mencoder.exe "C:\output1.mp4" "C:\output2.mp4" -o "C:\\combineVideo.mp4" -of lavf -lavfopts format=mp4 -vf harddup -oac copy -ovc lavc -lavcopts acodec=copy:vcodec=mpeg4:vbitrate=8000


any help would be appreciated, thank in advance.