I am getting the following warning message when opening my model:

Warning, Entity # has already had a material assigned.  The last assigned material will be used in the analysis.

Can you please suggest what could be causing this ?

 This warning is reported upon opening a STAAD model if it contains one or more members that have been assigned multiple materials.  For example, consider the following excerpt from the input command file of a model:

DEFINE MATERIAL START
ISOTROPIC STEEL
E 29000
POISSON 0.3
DENSITY 0.000283
ALPHA 6e-06
DAMP 0.03
TYPE STEEL
STRENGTH FY 36 FU 58 RY 1.5 RT 1.2
ISOTROPIC CONCRETE
E 3150
POISSON 0.17
DENSITY 8.68e-05
ALPHA 5.5e-06
DAMP 0.05
G 1346.15
TYPE CONCRETE
STRENGTH FCU 4
END DEFINE MATERIAL
CONSTANTS
MATERIAL STEEL MEMB 1 TO 16 17 18 TO 23
MATERIAL CONCRETE MEMB 17

As you can see, member # 17 has been assigned both STEEL and CONCRETE material properties.  Correspondingly, when this model is opened the following warning message appears:

To resolve this, you must edit the command file so no members are assigned more than one material.  In this example you could accomplish this by removing member # 17 from the STEEL material assignment as shown below:

DEFINE MATERIAL START
ISOTROPIC STEEL
E 29000
POISSON 0.3
DENSITY 0.000283
ALPHA 6e-06
DAMP 0.03
TYPE STEEL
STRENGTH FY 36 FU 58 RY 1.5 RT 1.2
ISOTROPIC CONCRETE
E 3150
POISSON 0.17
DENSITY 8.68e-05
ALPHA 5.5e-06
DAMP 0.05
G 1346.15
TYPE CONCRETE
STRENGTH FCU 4
END DEFINE MATERIAL
CONSTANTS
MATERIAL STEEL MEMB 1 TO 16 18 TO 23
MATERIAL CONCRETE MEMB 17

Once this is done, the warning message will no longer be reported when opening the model.