There is no name in "" for entry Lefever:2009

[whsun21]

This is BibTeX, Version 0.99d (TeX Live 2015/Debian)
The top-level auxiliary file: Thesis.aux
The style file: Biblio/gbt7714-unsrt.bst
Database file #1: Biblio/ref.bib
Warning--Empty string!
There is no name in "" for entry Lefever:2009
while executing---line 1628 of file Biblio/gbt7714-unsrt.bst
Warning--Empty string!
There is no name in "" for entry Lefever:2009
while executing---line 1628 of file Biblio/gbt7714-unsrt.bst
(There were 2 error messages)

Solution: change line 686 in Biblio/ref.bib
from

author={Els Lefever and Lieve Macken and and Veronique Hoste}

to

author={Els Lefever and Lieve Macken and Veronique Hoste}

bibtex thought there is an empty name "" between two and.