Error: there are 2 nodes that use LEIM located at (79, 0)

hello,

I'm using Stratix IV EP4SE820F43C3, and met an error I've never met before.

The error messsage is :

Error: there are 2 nodes that use LEIM located at (79, 0), but only one is allowed.

Error:Node f1_clk ~input drives the following nodes:

Error:Node M1120:U_M1120|f2_1120_ff1[1] uses the INCLK input at index 0 and placed at PIN AM25.

Error:Node i_vi_clk~input drives the following nodes

Error:Node vi:U_VI|o_vi_pdata[0] uses the OUTCLK input at index 0 and placed at PIN AP24.

My question is :

1, what LEIM is ?

2, what kind of reason result in this kind of error?

thanks

Mary