need help scd type1 query in sql server 2008

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SQL

I have a question about SQL Server.

Table: Dimemp

empkey |  empid | name |loc
1      |  100   | abc  |Hyd
2      |  102   | def  |chen
3      |  -1    | NA   |NA

Table2 : dimdept

deptkey  |  deptno  | deptname
1        |  10      |Hr
2        |  20      |ceo
3        |  -1      |NA

Transaction table: tranemp

empid    |  deptno   |  projectname |codes
100      |   20      |  test        |100
104      |   10      |  deve        |101
102      |   10      |  test        |107
106      |   40      |  sap         |103
101      |           |  Ca          |100
         |   10      |  manual      |201
100      |   60      |  quality     |100

Here the transedmp table data look into dimensions table if records match exact record then retrieve corresponding keys .if records not match then we need to fill defaule values -1 corresponding key value common columns for traneemp and dimension tables. empid(tranemp)=dimemp(empid) and tranemp(deptno)=dimdept(deptno) here I need to maintain scd type1 concept in Facttranemptable.

when I ran script at first time data look like in the below Based on tranemp table I want loade data in facttranemp and data look like below facttranemp table table structure like below : factkey is indenity(1,1),empkey--int,deptkey--int,projectname varchar(50),codes--int

factkey|empkey| deptkey| projectname   | codes  
1      |  1   |  2     |   test        |100
2      |  3   |  1     |   deve        |101
3      |  2   |  1     |   test        |107
4      |  3   |  3     |    sap        |103
5      |  3   |  3     |    Ca         |100
6      |  3   |  1     |   manual      |201
7      |  1   |  3     |  quality      |100

I tried like this:

insert into facttranemp
 select ISNULL(a.empkey, (select empkey from Dimemp where empid = -1)),
       ISNULL(b.deptkey, (select deptkey from dimdept where deptno = -1)), 
       c.projectname, c.codes 
from  traneemp c 
left join dimemp a on a.empid=c.empid
left join dimdept b on b.deptno=c.deptno

second day transaction table : tranemp data look like below

empid    |  deptno   |  projectname |codes
100      |   20      |   test123    |100 (Old)----here projectname changed test to test123
104      |   10      |   deve       |101
102      |   10      |  test        |107
106      |   40      |   sap        |105 (0ld) -------here codes changed from 103 t0 105
101      |           |  Ca          |100
         |   10      |  manual      |201
100      |   60      |  quality     |100
100      |   10      |  manual      |200 ( new) --new recordcame

Based on tranemp table I want loade data in facttranemp and data look like below

factkey|empkey| deptkey| projectname   | codes  
1      |  1   |  2     |   test123     |100
2      |  3   |  1     |   deve        |101
3      |  2   |  1     |   test        |107
4      |  3   |  3     |    sap        |105
5      |  3   |  3     |    Ca         |100
6      |  3   |  1     |   manual      |201
7      |  1   |  3     |  quality      |100
8      |  1   |  1     |   manual      |200

I am not able to implement scd type1 tranemp table with facttranemp and corresponding keys. I tried another way with merge statment like below

merge into facttranemp target 
       using( select ISNULL(a.empkey, (select empkey from Dimemp where empid = -1)) as empkey,
       ISNULL(b.deptkey, (select deptkey from dimdept where deptno = -1)) deptkey, 
       c.projectname, c.codes 
from  tranemp c 
left join dimemp a on a.empid=c.empid
left join dimdept b on b.deptno=c.deptno)source  on target.empkey=source.empkey and target.deptkey=source.deptkey
when not matched 
then 
insert ( empkey,deptkey,projectname,codes) values(source.empkey,source.deptkey,source.projectname,source.codes)
when matched then
update  set  
      target.projectname= source.projectname
      ,target.codes    =source.codes
;

That time I got error like below

Msg 8672, Level 16, State 1, Line 1
The MERGE statement attempted to UPDATE or DELETE the same row more than once. 
This happens when a target row matches more than one source row.
 A MERGE statement cannot UPDATE/DELETE the same row of the target table multiple times.
 Refine the ON clause to ensure a target row matches at most one source row, or use the GROUP BY clause to group the source rows.

Here I take unique combination empkey and deptkey .

I am not able to get expected result. Please tell me how to write query to achieve this task in SQL Server.