Why does XST bus elaboration cause name conflicts?
"WARNING:Xst:51 - Detected potential name conflict between port <a> and port <a_1> in unit <test>. Port <a_1> is renamed to <a_1_ren>, changing the interface of this block. Please update your source code accordingly or choose a different name."
To work around this issue, change the name of your output ports in System Generator for DSP, or use Synplify or Leonardo for synthesis.
For example, <a> and <a_1> should be named <a_out> and <a1_out>.
This issue is fixed in ISE 9.1.