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I was not able to reproduce the strange behaviour that you mentioned. I took your reference and created a biblio.bib file and used it in the following simple code:
\documentclass{book}
\usepackage{natbib}

\begin{document}

As was noticed in \citep{Amelin},

As was noticed in \citet{Amelin},

As ...

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Nobody an idea how to solve this issue

I have a BibTeX question. Let me first tell how I defined an article:
@article{Amelin,
Author = {Amelin, Y. and Krot, A. N. and Hutcheon, I. D. and Ulyanov, A. A.},
Title = {Lead Isotopic Ages of Chondrules and Calcium-Aluminum-Rich Inclusions},
Journal = {Science},
Year = {2002},
Volume ...