Non sono un esperto di R (su Windows poi...).
Prova a vedere se tra i pacchetti installati c'è già curl con installed.packages()
dovresti avere qualcosa tipo:
> installed.packages()
Package LibPath Version Priority
base "base" "/usr/lib/R/library" "3.2.3" "base"
boot "boot" "/usr/lib/R/library" "1.3-17" "recommended"
class "class" "/usr/lib/R/library" "7.3-14" "recommended"
cluster "cluster" "/usr/lib/R/library" "2.0.3" "recommended"
codetools "codetools" "/usr/lib/R/library" "0.2-14" "recommended"
compiler "compiler" "/usr/lib/R/library" "3.2.3" "base"
datasets "datasets" "/usr/lib/R/library" "3.2.3" "base"
foreign "foreign" "/usr/lib/R/library" "0.8-66" "recommended"
graphics "graphics" "/usr/lib/R/library" "3.2.3" "base"
grDevices "grDevices" "/usr/lib/R/library" "3.2.3" "base"
grid "grid" "/usr/lib/R/library" "3.2.3" "base"
KernSmooth "KernSmooth" "/usr/lib/R/library" "2.23-15" "recommended"
lattice "lattice" "/usr/lib/R/library" "0.20-33" "recommended"
MASS "MASS" "/usr/lib/R/library" "7.3-45" "recommended"
Matrix "Matrix" "/usr/lib/R/library" "1.2-3" "recommended"
methods "methods" "/usr/lib/R/library" "3.2.3" "base"
mgcv "mgcv" "/usr/lib/R/library" "1.8-11" "recommended"
nlme "nlme" "/usr/lib/R/library" "3.1-124" "recommended"
nnet "nnet" "/usr/lib/R/library" "7.3-12" "recommended"
parallel "parallel" "/usr/lib/R/library" "3.2.3" "base"
rpart "rpart" "/usr/lib/R/library" "4.1-10" "recommended"
spatial "spatial" "/usr/lib/R/library" "7.3-11" "recommended"
splines "splines" "/usr/lib/R/library" "3.2.3" "base"
stats "stats" "/usr/lib/R/library" "3.2.3" "base"
stats4 "stats4" "/usr/lib/R/library" "3.2.3" "base"
survival "survival" "/usr/lib/R/library" "2.38-3" "recommended"
tcltk "tcltk" "/usr/lib/R/library" "3.2.3" "base"
tools "tools" "/usr/lib/R/library" "3.2.3" "base"
utils "utils" "/usr/lib/R/library" "3.2.3" "base"
...
se c'è prova a rimuoverlo con
remove.packages("curl")