Error(1147): This module has already been defined - fortran

Question

I write a code that employs a module from another code (I didn't write it). When I use this module with the main program, it works well and shows no error. When I use this module with the subroutine it shows:
This module has already been defined
tried to summarize the code properly.
Here is reproducible code:
subroutine polygoneclipping(n,vtable0,vcoord0,BOUNDARYPOINTS0,NEWvcoord0,Unumber)
use SutherlandHodgmanUtil
only : polygon,sutherlandHodgman,edgeClipping
type(polygon) :: p1, p2, res
integer :: c, n
double precision, dimension(2) :: y1, y2
integer(kind =3) :: i, Unumber
integer(kind =3), dimension(40,1) :: vtable0
real(kind =3), dimension(40,2) :: VCOORD0
real(kind =3), dimension(4,2) :: BOUNDARYPOINTS0
real(kind =3), dimension(40,2) :: NEWvcoord0
!MAIN SUB PART
end subroutine
module SutherlandHodgmanUtil
type polygon
!type for polygons
! when you define a polygon, the first and the last vertices have to be the same
integer :: n
double precision, dimension(:,:), allocatable :: vertex
end type polygon
contains
subroutine sutherlandHodgman( ref, clip, outputPolygon )
type(polygon) :: ref, clip, outputPolygon
type(polygon) :: workPolygon ! polygon clipped step by step
double precision, dimension(2) :: y1,y2 ! vertices of edge to clip workPolygon
integer :: i
!MAIN SUB PART
end subroutine sutherlandHodgman
subroutine edgeClipping( poly, y1, y2, outputPoly )
type(polygon) :: poly, outputPoly
double precision, dimension(2) :: y1, y2, x1, x2, intersecPoint
integer :: i, c
!MAIN SUB PART
end subroutine edgeClipping
function intersection( x1, x2, y1, y2)
double precision, dimension(2) :: x1, x2, & ! points of the segment
y1, y2 ! points of the line
double precision, dimension(2) :: intersection, vx, vy, x1y1
double precision :: a
!MAIN FUNC PART
end function intersection
function inside( p, y1, y2)
double precision, dimension(2) :: p, y1, y2, v1, v2
logical :: inside
!MAIN FUNC PART
end function inside
end module SutherlandHodgmanUtil
What is this error and why it appears?
I will happy with any help and forgive me for the long code.

Answer 1

If I run your code I get the following error message
a.f90:3:7:
3 | use SutherlandHodgmanUtil
| 1
Fatal Error: Cannot open module file ‘sutherlandhodgmanutil.mod’ for reading at (1): No such file or directory
Explanation for that: In Fortran there are no automatic forward declarations.
This means that the subroutine which tries to include sutherlandhodgmanutil fails as the module has not yet been defined.
Solutions:
Define the subroutine after the module.
Preferred way: Create a separate file for the module and compile the module first.
Your error message
This module has already been defined
probably results as you already have compiled the module SutherlandHodgmanUtil beforehand?
Either from a different file or different test run?
But that's just a wild guess..
By the way, I had to change your code as integer(kind=3) was not available on my machine.
In general it is preferred to use machine independent kind defintions, e.g. check out use iso_fortran_env, only: int32, real32.