@Gunnar: You are right, I tested ibus-sunpinyin (Chinese sunpinyin), using it input some Chinese words in webapp, It dose work well, doesn't have this problem.
Input Chinese word in webapp via ibus, in summary:
ibus-sunpinyin sunpinyin engine for ibus, version: 2.0.3-5build1, works correctly
ibus-pinyin and ibus-bopomofo in pinyin engine for ibus, version: 1.5.0-1ubuntu1 , words are duplicated
@Gunnar: You are right, I tested ibus-sunpinyin (Chinese sunpinyin), using it input some Chinese words in webapp, It dose work well, doesn't have this problem.
Input Chinese word in webapp via ibus, in summary:
ibus-sunpinyin sunpinyin engine for ibus, version: 2.0.3-5build1, works correctly
ibus-pinyin and ibus-bopomofo in pinyin engine for ibus, version: 1.5.0-1ubuntu1 , words are duplicated
My system: Ubuntu 14.04 64bit
So, does it mean this issue belongs to im-config?