Apacite package with "natbibapa" option includes initials of one author

Question

When citing Jakob Nielsen's works with apacite package and bibliography style (with the option natbibapa), the citation call-out is generated like this:

(J. Nielsen, 1993; J. Nielsen & Molich, 1990)

This is not what I intend, as APA should not include the author's first name as an initial. It should be like this:

(Nielsen, 1993; Nielsen & Molich, 1990)

This is how I've done it in LaTeX:

\citep{Nielsen1990, Nielsen1993} 

I am also curious about why the sequence is inverted to how I wrote in LaTeX.

BibTeX entries, generated by Mendeley:

@inproceedings{Nielsen1990,
address = {New York, New York, USA},
author = {Nielsen, Jakob and Molich, Rolf},
booktitle = {Proceedings of the SIGCHI conference on Human factors in computing systems Empowering people - CHI '90},
doi = {10.1145/97243.97281},
isbn = {0201509326},
pages = {249--256},
publisher = {ACM Press},
title = {{Heuristic evaluation of user interfaces}},
url = {http://portal.acm.org/citation.cfm?doid=97243.97281},
year = {1990}
}
@book{Nielsen1993,
archivePrefix = {arXiv},
arxivId = {arXiv:1011.1669v3},
author = {Nielsen, Jakob},
booktitle = {Usability Engineering},
doi = {10.1145/1508044.1508050},
eprint = {arXiv:1011.1669v3},
isbn = {0125184069},
issn = {10772626},
number = {3},
pages = {362},
pmid = {18369261},
title = {{Usability Engineering}},
url = {http://www.useit.com/jakob/useengbook.html},
volume = {44},
year = {1993}
}

MWE:

\documentclass[12pt, twoside]{book}    
\usepackage[natbibapa]{apacite}
\bibliographystyle{apacite}

\begin{document}
\citep{Nielsen1990, Nielsen1993} 
\bibliography{Bibliography/bibliografi,Bibliography/library} 
\end{document}

Answer 1

This behaviour is expected if there is another author with different first name but same last name in the bibliography. Otherwise authors with the same last name would not be unique.

The OPs bibliography contained works of both "J. Nielsen" and "L. Nielsen". LaTeX quoted with the initials to make them distinguishable.

This behaviours also occurs, if there is an author of the same last name in the bibliography, but with a mis-spelled first name, an abbreviated first name or 2 autors with the same first and last name, but only one with a middle name. Bib(la)tex doesn't know they're the same and abbreviates their first names when they're cited.