We did not jump any gun...!!!
There are two daughters Johanna Margaretha born to Abraham Gerardus Stuffers and Margaretha van den Berg.
The first was born on 02 June 1848. She died four months later, on 19 Oct 1848. That is the one whose death you found.....
Then in 1850 they had another daughter, who they named Johanna Margaretha as well. This was not an uncommon practice in The Netherlands, where children are often named after their grandparents or other family members.
And it is this 1850 Johanna Margaretha who went to the USA.
So you see, all is still well.