Multiple mlt.fl does not work well if the termvectors is repeated

for an example：

my data is：

{ "id":"100079750", "title":" "I like cat, don't like dog", "tags":["cat"], "desc":["my cat photo"] }

by the way title and desc's Tokenizer is IK.

and filed tags' type is text_ws.

while using mlt.fl=title,tags,desc with parameters debugQuery

the result shows：

"interestingTerms":[ "desc:my",1.0, "desc:photo",1.0, "desc:don",1.0, "title:dog",1.0, "desc:cat",1.0, "title:like",1.0],

"debug":{

"rawquerystring":"id:61",

"querystring":"id:61",

"parsedquery":"desc:my desc:photo desc:don title:dog desc:cat title:like", "parsedquery_toString":"desc:my desc:photo desc:don title:dog desc:cat title:like",

......

look at the word cat

it appears in field tags, desc and title,

but the result shows  the word cat only used in field desc and was ignored in field tags and title.

Finally, I found the reason when the word is repeated in more than one field.It will only be used in one field to do the work.

otherwise sometimes word is only in field tags, but while doing the mlt, the word was shows as other field such as title or desc, in fact there is never appear in these fields!