I'm saying kaddish for my mother-in-law. Occassionally, I walk into shul and the congregation is already in the middle of the Rabbi's Kaddish (say at Y'he sh'may raba"). Can I join in that kaddish at all, or have I lost out? If I can join, do I quickly say the parts I missed and try to catch up, or do I pick up from where the other mourners are at? What if I arrived on time, but I'm at a different point in the service?
This Jewish Mourning Guide makes the following points about Kaddish – please see his references:
Ashkenazi Poskim (torah authorities) hold that L’Chatchilah (preferably), only one person should say each Kaddish (prayer said on behalf of the deceased). The Rema and Bi’ur Halachah give the order of precedence. To avoid fights, many places allow all the Avelim (mourners) to say Kaddish together. However, they must say it together word-for-word,[Gesher HaChaim item 12, p 299] …
…(the rest of the Mourning Guide needs careful study).
If you come late it will not be possible to say the Kaddish together with the others word for word. So it seems that you should give up on that kaddish.
When I was saying Kaddish, someone told me how many kaddeshim I had to say. My Rov told me that it was only necessary to say one Kaddish per day.(!)