SAP Portal to ECC SSO fails at first attempt,but working in second attempt

Hi Experts,

We have configured SSO from SAP EP to SAP ECC system. Below points are already checked.

1) Certificates are exchanged between SAP Portal & SAP SRM

2) Trust Established to SAP ECC from SAP Portal

3) System Object for SAP ECC Created and Connection Test is passed

Now, while perform the testing of SSO from System Admin --> Support --> Test and Configuration Tools --> choose transaction --> Selected ECC system alias and mentioned common t.code (ex: SMEN) and clicked on Submit

We ended up with netweaver logon screen. If we close that and repeat the same steps for the second time, SSO is working. It means, without asking user id and password, SAP ECC Home Page appeared.

Kindly help me to identify this issue. What is disturbing the SSO for the first attempt ? and how its working on the second attempt? Kindly provide some light on this issue.

Thanks,

Nisha